What is the name of the file that my emails are stored in on my computer?

There are many. So perhaps you might want to share why you are asking. This is a topic on which hours if not days could be spent without much trouble.

Thanks for responding. When I open Thunderbird, nothing appears in my inbox. I have to manually download my messages and normally that takes several tries. Once I get the messages down loaded I want to be able to copy or save them permanently on my computer so I don't lose them.

Okay, I seriously mean no offense, but your statements are so open that we could discuss at length, as Matt indicated. :) So, seriously, tell us your intent. For example, one solution to your issue is to do daily backups. Or maybe you just want to save a few important messages. In truth, once you see a message, it is already on your PC, but that doesn't necessarily mean it's where you can find it. Thunderbird is terrific, but it wasn't designed to do permanent stuff. Again, for example, I use a backup service (free, there are many) that runs daily and copies my selected folders to the cloud. Free. Daily. I also do a full backup PRIOR to any thunderbird upgrade. Why? If there are problems, I can easily restore. Rarely needed, but practical. I also do a full hard drive backup twice a month. Hey, you never know. :) So, tell us your intent, and we will offer what we can. And you're very welcome here. :)

Ok, let us start with what you have said so far.

Thunderbird should always open with what was stored in your inbox the last time it was opened. If that is not the case then I think we have a problem, but most likely not one of Thunderbird making.

What security software do you have installed. (antivirus, VPN and anything that prurorts to make your system faster, leaner or in truth usually more unreliable and less stable. I class CCleaner as one of the worst culprits for over promising and misleading claims. But it has it's adherents. I have it installed and it is one of the most virus like product I have ever tolerated. BTW I do not allow it to change anything, I have it so I can warn others.) Is your account configured in Thunderbird as IMAP or POP. POP is a permanent protocol. Once you have mail locally you have it forever it only gets new mail usually form the inbox. IMAP is syncronised to the sever and any interruption to connectivity will see the existing downloads cleared as they can not be validated as current with the server. These mails are sometime re-downloaded because of changes the antivirus makes as they are scanned. And perhaps the most important of all. What error messages are you being presented with. It would be very unusual to have an empty inbox with out some sort of message about connection problems

I do recommend no scanning of mail or encrypted connections in the antivirus product while diagnostics occur. This includes any scam or spam filtering external to Thunderbird (often another over extension of Antivirus products these days.

Thank you for responding. The problem has been solved. First I exported my files to another device to back them up; Then a friend found that the Thunderbird App was searching for folders that had been "Subscribed". The "Subscribe function is under the File tab. I had never used the function but for some reason it was now active. Once I "Subscribed" to the folders I wanted to use, all the subscribed folders, including my inbox, were populated with the proper emails when I opened my Thunderbird app.

Thank you for responding. The problem has been solved. First I exported my files to another device to back them up; Then a friend found that the Thunderbird App was searching for folders that had been "Subscribed". The "Subscribe function is under the File tab. I had never used the function but for some reason it was now active. Once I "Subscribed" to the folders I wanted to use, all the subscribed folders, including my inbox, were populated with the proper emails when I opened my Thunderbird app.

Subscribed is a function of the IMAP mail protocol and has existed and been a part of Thunderbird for more than 20 years. Subscription is automatic when an account is created. That you had to re subscribe indicates a problem in and of itself. It sounds like something corrupted the local storage that included the accounts subscriptions. Normally I would suggest deleting the foldertree.json file unless such a thing happens twice. So good you got it working. Now to see if it recurs.