Try booting into single mode
, then execute "grep /etc/passwd -e <username>", which should return the line with the username you entered, meaning the user is known. If so try executing "less /var/log/messages" and scroll to about the time you tried to login. Then if there are any error messages about logging in I guess they will be prefixed with either "login" or something starting with "pam".
If the user is not known in passwd, then you'll have to add it again, if the log shows errors, try posting 'em here.