Unable to login via SSH with password - can login with ssh key and can su

This is because sshd has registered too many logins with the incorrect password.
To reset the incorrect password count, run the following command:

pam_tally2 --<user> --reset

This is configured under /etc/pam.d/sshd

Original article at:

http://www.tecmint.com/use-pam_tally2-to-lock-and-unlock-ssh-failed-login-attempts