The hard way:
You can also hit <ctrl><alt><F1> and exit to a terminal. Login as root if necessary. Then run 'ps -A'
Look for your login program, typically gdm or kdm, type 'kill ####' (the process number you got from the ps command).
At the very least, this should reset gdm/kdm, but it might kill them outright. If it does, type 'kdm' or 'gdm' to start them back up. You should be good to go at that point.
An easier way is to logout, then hit <ctrl><alt><backspace> This should restart your X session.