Password Failure for one user

 I am trying to track down why one user all of a sudden can't log into a machine. He can use his LDAP passwd through our network, but gets an immediate login failure on a particular machine. I can 'su' to his account, from my account using his password, but when I try to log into this machine remotely, I get the failure and it terminates the sessions. There have been no changes to the system at all. I am looking for step to troubleshoot this issue. Thanks.