Boot the run level 3, then try running /etc/pam.d/authconfig (not positive on the path, don't have linux running where iam) but when you get it running it'll have a blue screen wizard. Set it not authenticate off any servers, just linux shadow passwords. Then restart, see if it boots up ok. If that doesn't help, or you use pam.conf and don't have the above dir, then post more information about the problem.