First, I have no idea what the problem is. My suggestion is to log onto your online account to verify that the attachments are there and that you can download them. That confirms they are arriving.You might then ask the sender for more info on how he sends attachments and share the problem you're having. That may provide more info to share here on resolving this. Thank you.

Thanks for helping brainstorm the problem!! I really appreciate it. When you say to log into my online account to make sure the attachments are there you are referring to my email client, correct? I use Zoho. Yesterday, I was resorting to logging into my Zoho app from my phone and then forwarding myself the emails from my business partner. I was able to receive the attachments this way. He also uses Zoho, so theoretically Zoho is not the problem. It would be one thing if only new emails since yesterday were not allowing me access to attachments because that would tell me that something in his settings got messed up. However, it's especially puzzling because I cannot access ANY attachments that he has ever sent me. I'm pulling my hair out over here!

Does this problem occur when receiving mail from non-Zoho users? If not, then Zoho may be part of the issue.

The sender might be using a mail app that fails to correctly encode the attachments (common with Apple mail apps). Look at the message source (Ctrl+U) and the Content-type header for the attachment. If it's something like application/octet-stream, instead of application/jpeg (for pictures), TB might not be able to decode it.

Settings/General/Config. editor, toggle mailnews.display.show_all_body_parts_menu to true to display All Body Parts in the View/Message Body As menu. Press Alt if the Menu Bar with View is hidden. Select All Body Parts; do you see anything different in the attachment pane of a received message?

If the problem can't be resolved with this sender, you could ask them to upload files to a storage space like Google Drive or OneDrive and send you the links to download.

The sender is using Zoho. I also use Zoho and tested it yesterday to send myself some files and it worked fine. I even forwarded myself one of the emails from my business partner and the attachment worked then, but doesn't work if coming from him. His emails worked perfectly for the past several years using this system and just all of the sudden yesterday everything went kerplunk kerpluey. I don't think the problem is the way it is being sent. Because the emails from months ago all came through without any issues and are just now not allowing me to access the attachments :(

Yesterday, I toggled what you suggested here: Settings/General/Config. editor, toggle mailnews.display.show_all_body_parts_menu to true to display All Body Parts in the View/Message Body As menu.

I also noticed another thing - on one of the emails my partner sent over today, I can't see any of the body/text of the email either.

I don't think it matters that the sender uses Zoho or any other service. It's probably more dependent on their mail app. What's the file type of the attachments? If they once arrived correctly, what has changed since on your system, e.g. security software? Is any 3rd-party app on the system scanning/interfering with email?

The files are PDF's. As far as I know, I haven't changed anything on my computer. I think I have Mcafee doing something on my computer - I get notifications here and there from it. Sounds like you think that could be the culprit?

Run Windows in safe mode to bypass startup apps like antivirus, or remove (not disable) McAfee, to test for possible AV interference. McAfee is a well-known source of mail issues; at the very least exclude the TB profile folder from AV scanning.

https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#McAfee

You can force a re-download of a message in an IMAP folder by opening the source (Ctrl+U). Test also by sending yourself a pdf attachment.

Thank you so much for your help! After seeing your message from before about maybe McAfee causing it, I clicked to run a scan with Mcafee and then updated/restarted my computer. I opened up Thunderbird and voila!! It works now. Who knows exactly WHAT fixed it (I don't think McAfee actually did anything when I scanned it), but I am SO glad all is well. I appreciate all your efforts. Thanks again!!

I don't think McAfee actually did anything when I scanned it

No but it probably updated itself prior to the scan.