A little more info would be very usefull. Try reading the xsession.error log, which is in your home directory. The file name is .xsession.......
You will have to log into console mode. Press CTRL+ALT+F1 to switch to console mode. Login there. Once inside, write less .xsession (press TAB here, and be sure to write the . (dot) before xsession)
Post here, what that gives you. Or provide some more details on how the system "locks up"