Please see https://support.mozilla.org/en-US/kb/sending-and-receiving-messages-thunderbird

Sorry to be a problem here for you guys..When I go down to outgoing server (SMTP) , your instructions say make your changes here ( what changes do I need make ? I did change port to 25 , I am using win-7, security is SSL/TLS , server name is smtp.googlemail.com , normal pswd and my correct email address )..should I change the port no.? Help please..this is so confusing..

I changed the out port to (465 default) and still no help..I rebooted also..Now I have changed it back to 25 and nothing..Help..

You need to make sure your settings match what Gmail servers require - and the authentication/security settings that your Gmail account requires. Check all the settings in both Outgoing and Incoming. Start with whether IMAP or POP - the settings are different. Then search online for the current settings required by Gmail for IMAP/POP/SMTP.

Then there's more detail of where to enter those Gmail setting in Thunderbird here - https://support.mozilla.org/en-US/kb/configuration-options-accounts

Agnes..Thanks a bunch..I agree completely..I discovered this yesterday the hard way..This link you shared took me down the rabbit hole much more than all the articles I read that said , put it here , what do I put there.? This is where it became frustrating with the links that did not go into detail any more than they did.. Well , I have a laptop that has Thunderbird on it so I opened the in/out server settings and found my problem..The upgrade changed settings in my server , that I never look at , but this time I did and I also copied every setting so if this happens again..I think in some cases it may be wise to go back thru and set up a new email and let Thunderbird finish..So glad to get my Thunderbird back.. Gary Bass