Maybe if you share your error messages or whatever, someone here may be able to help. I regret you are having difficulties. Thunderbird is very sensitive to antivirus programs that tend to interrupt updates.

I didn't post the error message because there is none. It just keeps working to download forever, but no mail is downloaded.

The account may be corrupted. I suggest trying to send a message to see if it works. You might also attempt to repair by clicking settings>privacy&security, click saved passwords and then show passwords and then highlight the incoming and outgoing server, rightclick and delele. That would remove passwords and, if that helped, you will be prompted for password to retrieve. That may not do it, but the connection isn't working. Good luck.