So this is a Gmail account, right? Have you set it up in Thunderbird as POP or as IMAP? Do you also access this account from some other device or application?

It is an Imap. It has been the same Gmail for years and was working up to yesterday. it is up to date and I have restarted it and the computer. It does everything like it is going to check mail but says there is not. I can see there is if I go to Gmail. I can send mail. I have been getting a security certificate pop up for a couple of weeks that I have been telling to continue anyway. I get the same one on e out going mail .

Any error messages you get would be important to diagnose the problem. You need to report that info more accurately by either providing screenshots of them, or typing here what is it exactly that they say.