I need a real linux genious for this one :

When the X boot up screen is displayed, I try to log in as root. I type the password in and get a box that say 'authentication failed'. The weird part about this is, when I type in the wrong pass I get no box, only when I type in the correct pass. Also, I have another user on the same PC, when I type in his password I get the same box.

What has possibly gone wrong here!? The correct password won't let me into my system. If anyone has had this happen before please dear god help!