that is generic login failure dialog. Thunderbird did not forget the server refused. Two quite different tings.

My first reaction is what has changed with century link. do your settings in Thunderbird match those published by century link. https://www.centurylink.com/home/help/internet/email/how-to-set-up-your-email-server-settings-to-go-through-pop-and-smtp.html#server-settings

Has there been some sort of outage at centurylink involving mail?

Do you have a VPN which hides your connection via certurylink from their servers that might cause them to refuse to allow you to login (it is more common these days, both providers refusing mail access unless you use their ISP services and VPN's that just turn themselves on randomly.)

Server settings were checked and still as published. No VPN (unless some sneaky ugly software has invaded). Password has not been changed in the recent past. Was working well previous evening, computer not shut down overnight, no power failures, etc. This morning it was popping that wonderful dialogue. Have checked the webmail site for century link and email activity appears normal and functioning.

Will not be surprised if it is a century link issue, but no real pointers that direction. Have deleted and manually re-entered password on webmail to absolutely confirm correct and reentered in the dialogue pop-up, no joy. Software and computer rebooted, but have not deleted/disconnected "inbox" or deleted/reinstalled Thunderbird---and only started using a couple of weeks ago as Microsoft has seriously dropped something stinky with "new" outlook and barriers to installing "old" outlook.

whenever the server becomes unavailable or is having problems, this is the error you will get. It will clear when it is resolved. centurylink has many problems. It used to be my ISP, but I got tired of the errors. Especially when their server went south and they refused to move my account to a different server after telling me they were not sure when they would replace the server. You would think your account would float to whatever servers were the backups to that server automatically. After more than a month of no email, I left them for good. There's a reason at work we used to call them centurystink.