https://support.mozilla.org/en-US/kb/cannot-receive-messages

Hi Dave. Thanks for marking the solution. What specific change did you need to make?

DaveAsk said

Since Thunderbird updated to 38.2.0 it will not send the password. It connects to the email server but does nothing after that. I cannot get any mail. I am able to send mail with no problem. I am using GMX.COM as an email server.

I had to reload Thunderbird to get it to work.

I had to download and reload Thunderbird to get it to work. That solved the problem.