We have been running 3.3.1 successfully on an ISP for some weeks. Today login failed and four new passwords were rejected.
Have you tried requesting a new password? Have you also checked that Caps Lock is not on?
Alternatively, you may need to go in to the phpList database and manually change your password via phpMyAdmin
Quite right, Dragonrider, stupid mistakes department! User name on login screen defaulted to Email Name instead of Name and I missed it.