Go to your email providers web mail page and confirm that you have a working username and password. If not, fix them there. When you have a working password go into Thunderbird and delete the saved passwords and then close Thunderbird. When you restart Thunderbird you will be asked for a password. Enter the new, working password. Here is how to remove and replace the stored passwords. http://thunderbirdtweaks.blogspot.com/2011/07/pssswords.html If you still have problems it might be incorrect server settings. Get a current list from your provider and compare those with what you have in Thunderbird.