The error message is coming from your email provider. it may be a setup misconfiguration, or your email provider has specific requirements for passwords on PCs (GMAIL is an example of that). if you post full error message and screenshots of your incoming and outgoing server settings panes, someone here may have suggestions,

This is for a Verizon.net email account. The phone email & AOL emails work, just not the Thunderbird email account. Worked fine up to a few days ago & now I just end up in a continuous loop to enter password & when I do, it doesn't work.

Not sure what server settings you are using - maybe it's worth posting animage of your current settings. But first please do the following.

Verizon/AOL Your normal password gets you access to your webmail account via a browser. However, you cannot use that password in a third party email client such as Thunderbird. You have to access your webmail account and create a special app password which you use in Thunderbird.

Go to the following link and create an 'app password'. https://help.aol.com/articles/Create-and-manage-app-password

Then in Thunderbird you have to edit your current stored password. Settings > Privacy & Security Scroll to Passwords section click on 'Saved Passwords' click on 'Show Passwords'

You will have two lines for the account - one is incoming so either 'mailbox://...' or 'imap://....' The other will be an outgoing - smtp://.... Right click on each line and select 'Edit Password' Completely clear the contents and then carefully enter the new app specific password. When finished doing both incoming and outgoing passwords - click on 'Close' Then exit Thunderbird. Wait a few moments for background processes to complete - start Thunderbird.

Firewall you need to make sure that the Thunderbird program is set up as allowed.

VPN. Also check you do not have any VPN switched on because server do not like it as it attempts to hide IP addresses so server thinks you are a hacker and blocks access. I have come across several people who did not realise they had a VPN enabled. Many Anti-virus products now offer a VPN and in some cases they auto enable it. VPN needs to be switched off OR you have to create a split tunnelling and set up Thunderbird to use it.