You need to check first using webmail whether the messages are arriving at the server and it's Thunderbird that doesn't see them, or it's the incoming mail server that's not receiving the messages in the first place, in which case the problem has nothing to do with Thunderbird…

What type of account is this (POP/IMAP)? Which is your email provider?

Read the following article in the meantime to see possible causes of the problem:

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

In 2016 Midco acquired WOW! cable, internet and phone. I kept my wowway email account and also created a midco account. The wowway email worked fine until a week ago. It now can send messages but not receive them. It is a POP account. I chatted with a midco tech, who said "yes, midco acquired the wowway accounts, but not yours." Obviously untrue, but I think it means that the account has disappeared from midco's records. In server settings, I adjusted the Port number to agree with the functioning midco account -- no success. and I tried changing the connection security to "password, transmitted insecurely".

When I click between accounts, in the lower left corner both accounts say "loading messages" on both accounts. When I click the cloud in upper left, midco says "No messages", but wowway does not say anything.

Thank you DavidGG for responding.

I tried the suggestions in the link below a second time, and solved the problem. Thanks!

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

Great! Do you know which exactly of the things mentioned in that article is the one that fixed your problem?