Can you supply more information about what led to this ? For example - Is this a full installation which has worked ok before ? If so, what changed ? If it is a new install what options did you select ? Any error messages while booting ?
At the login screen, have you tried accessing a terminal session instead of logging into the GUI ?
Press Ctl/Alt/F2 at this point and see if your login works. If so you may be able to determine why the GUI login doesn't complete.