Are you trying to log in as root? If so, log in as your main user, since root login has been disabled (that's true for the login screen as well as for SSH if it's configured sensibly).
If (what I rather think is the case, considering your presence on this forum) this is a new problem (in the sense of, it happened recently without previous warning), you may be in trouble (if someone was able to change your password, you've been either hacked or tricked). You'll have to look for a way to start your server in single-user mode (refer to
this link) - or use a rescue CD like
SystemRescueCD or
Finnix. But before doing the latter, you'll have to read your way through something like "How to use
chroot"...
M.
