AD login problem

I have had a MacBook Pro (10.5.3) bound to our AD domain for the last few months. All has been well. Main user been using a mobile account. Yesterday the user was unable to login. Small grey spinning wheel to the right of the login window would spin for a while and then the window would shake it's head as if to say "wrong password" I tried logging in to my AD account (which I had set up a number of months ago) same problem. Logged into a previously set up local account with no problems. I tried unbinding and rebinding = same problem I tried logging in as a user who had never touched this machine before. Success. Account was set up and instant access. I logged this user out and then back in again = same problem occurs - spinning wheel and shaking head. Now this account is also inaccessible. I updated to 10.5.4 = same problem Anyone any ideas?? Thanks Gordon