I suggest your first priority be to establish a backup plan now. If you wait until you possibly restore everything, you may lose the urgency of preventing a future issue. Software always crashes into oblivion without advance notice. And check that the restore works. A common problem I see are users with backups that, when restored, do not work.

Now, if the account is IMAP, log onto your online account. That will give you immediate confirmation on whether your messages are lost. if POP account, I suggest these steps:

1. click help>troubleshootinginformation
2. scroll down left side to 'profiles' and click 'about:profiles'
3. if there is more than one proflle, click to open the one not in use, and click the Mail folder and the then POP account folder. My hope is that thunderbird upped to another profile and the POP account is still there. If unsure, post screenshots of what you see there. What you want to see are files such as inbox, inbox.msf, sent, sent.msf, etc.

I am including a same backup strategy here:

BACKUP

1. ensure thunderbird is not running
2. enter %appdata% in search window on task bar and press Enter key
3. this should open windows file explorer in Appdata\roaming folder
4. highlight the Thunderbird folder and copy to external media

RESTORE

1. ensure thunderbird is installed and NOT running
2. enter %appdata% in search window on task bar and press Enter key
3. delete the thunderbird folder
4. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there.
5. start thunderbird and all should be there

This is probably going to sound really stupid but where do I fine the 'help' option even? Are you referring to on the thunderbird desktop (if so where?). Equally, 'log on to your online account', where do I do this and what online account? Is there a version of thunderbird online that I am unaware of? I'm sorry but I am not tech savvy, I guess I'm just incredibly confused as to why emails associated with my account would vanish in totality. Also, additional problem, can't send any emails, I get an error message which I will attach to this reply. I have put in my password when opening thunderbird and emails seem to come in, but I haven'tt been able to send any since this whole issue arose either.

The help menu is located on the menu bar or on the hamburger menu in upper right. I confess to being surprised that youve used Thunderbird for years and never located the menu. I was glad to share that info. By the online account, I was referring to the web access if account is IMAP. For example, if account is at gmail, the web access is gmail.com. Seeing the word 'relay' usually means that the email address of the SMTP setting is different from the email address of the account. It may help for that if you post screenshots of your server settings pane and of your SMTP server settings pane. thank you.

Straight up never paid attention to the three lines registering as a hamburger menu, always thought the 'menu' was the account settings one - trust me I feel like an idiot in regards to that, I'm only in my 20s so I had my account when I was even younger and not super tech savvy For the online account thing, I use a custom domain(? if that's the right word) via namesco and have tried logging into their webmail with no luck as to having any past emails, it is identical to my desktop in that sense. EDIT: I can however send emails there at least, though they turned up in the spam folder haha. When I test the server settings on desktop TB everything says the connection is fine and I believe all of my stuff is set to default, I'll try and have a closer look later.

Trying to go through settings everything says IMAP, but I don't know whether it has always been that or not. Looking at the folders (the users/appdata/roaming/etc etc) they pop3 host in the name and there are multiple folders, most of which with keysmash names. I looked at the mail files through notepad and the largest ones (only about 11k kb) look identical, even past the formatting text, the few chunks I can make out as emails are the same. When I synched the thunderbird desktop with the phone app would that have transferred the information/ could that have been the cause of this? Aso would mozilla possibly have anything on their end?

sorry if this is all over the place, the winds have been knocked out of my sails with this issue as it has been devastating and I am very pessimistic in recovering anything.

EDIT2: thanks for answering my questions and helping so far, I really do appreciate it even if I'm slow with understand these things

Also thank you for helping me out, forgot to add that in my response. I really appreciate it.

Well, there are sharper tools in this shed than I, but I will assist where I can. I understand the frustration.

First, you need to have the ability to create screenshots in JPG or PNG format that can be uploaded here. Windows has the ability to do that, but my fingers always stumble with that tool, and I prefer a separate, stand-alone program for screenshots: Here are two FREE ones that are relatively easy to use. The steps in using either of them is to 1) start the utility, then 2) switch view to the screen where you want to capture part of the screen, then 3) switch back to the utility and press the 'capture' button, and then 4) drag the mouse to capture a rectangle area and save it. https://www.reasyze.com/ look for 'sniptool' My favorite, because it easily lets one add arrows or text, is at https://portableapps.com/apps/graphics_pictures/fscportable

Next, do this:

1. click the three-line menu in upper right (often called the hamburger menu) and select 'account settings'
2. highlight the account in left side bar to ensure all screenshots are for that account
3. now, click 'server settings' in left sidebar and do a screenshot of the right side of screen.
4. now exit back to main Thunderbird screen
5. next, click the help>troubleshooting menu item
6. scroll down to 'profiles' and click 'about:profiles'
7. do a screenshot there.
8. post back with both screenshots

I infer that you logged on to website, checked email there, and found no messages in sent folder or in inbox. Please confirm. Thank you. I await your response. AND, some others may respond also to this thread, so you may get further assistance.

I am so sorry for the ridiculous wait on this response, I've been fighting with thunderbird and struggling to access this account (seeing as thunderbird sends an email code but then it was refusing to receive new emails...again......) it started behaving again a few days ago although still can not send emails, joy! And I did log in via webmail directly and there was none of my past emails or folders or anything there either, which had been my last true hope that anything could have survived. Hopefully the screenshots are understandable.

The fact that you have no old mail folders via webmail confirms that your lost mail account was POP, as that would have local on your PC. Your new setup, I see, is IMAP and that protects you from losing messages if the PC crashes. I notice there is just one profile. Just for curiosity, I suggest confirming there is not a OLD profile buried there. You can do that with these steps:

1. click help>troubleshootinginformation
2. scroll down to 'profile folder' and click 'open folder'
3. you will now be in Windows File Explorer
4. back up one folder so that you are looking at the Profiles folder itself. Do you see more than one profile in the profile folder? If so, look in the one that is not have the name of the one in your screenshot. See if there is anything in the Mail folder or the Mail\Local Folders folder. Let me know.

Ok, those suggested steps are just to confirm that all messages were, indeed, lost. Looking forward, it seems you're making progress. I understand you can receive mail but not send it? Please post complete text of error messages and a screenshot of the SMTP server settings pane. .You can locate that by clicking 'account settings, then highlight the account in left sidebar and click the 'edit outgoing server; button in lower right. See attached sample. With screenshot of error message and screenshot of SMTP server settings, my hope is that I, or others here, may have suggestions for you.