Incorrect server settings or an overactive antivirus software can cause password errors. Since you seem to know your password, it works in webmail, I would go to Tools-Options-Security-Passwords-Saved Passwords and make sure the correct password is stored there. Even if it looks correct you could remove it, exit Thunderbird and reenter it when you start Thunderbird again.

After that get a current list of server settings for your provider and check what you have in Thunderbird. They might have changed something and you need to update your settings.