My IMAP/SMTP login from Thunderbird stopped working

After working fine for several years, yesterday my Thunderbird software stopped collecting new messages from the AT&T IMAP server, but only on some of my email accounts. Any attempts to refresh my email yields this message: "Login to server imap.mail.att.net with username *****@pacbell.net failed." I have re-entered the correct password multiple times, but to no avail. Using the correct password, I can readily log in to the web email, but this is NOT how I need to access my mail! And I cannot send email from my main account using Thunderbird, with similar messages. I have double-checked that my server settings are according to the AT&T documentation, and rel-entered the correct password several times, with no luck. I have had similar problems for a long time on some of my secondary email accounts, but never solved them. But now this is happening on my primary email account, so I really have to figure it out. Is this a Thunderbird problem, or something with AT&T/Yahoo? PLEASE HELP.