Cached Credentials on Laptop Erased While on Trip - Couldn't Get into Laptop

The owner of our company uses a ThinkPad with Windows XP Pro connected to Windows Server 2003 when in the office.  He was recently out of the country for a couple of weeks and near the end of his trip, he was no longer allowed to log into his laptop.  He was getting a "Domain not Found" message.  Over the phone, I had him try to login with the server Administrator account and he was able to do this without error.  The cached credentials for his account however appear to have either expired or were erased/corrupted?  

When he got back into the office and connected his laptop to the LAN, he was finally able to login.  We then unplugged his laptop from the LAN and tested his login with cached credentials and it was working fine again.  This was a major problem, since he couldn't use his laptop at all.  He is very concerned that this will happen again and since he is out of state or country for at least 2 to 3 weeks a month, it is not possible for him to always come back into the office and login to the domain.  Does anyone know how this could have happened?  Note, someone that was with him from the company did try to login to his laptop with their account (an account that was not cached) and were obviously unable to do so.  He has not changed his password, installed software, etc. recently.  Is it possible that the person trying to login caused the problem?  Also, is there a workaround if you lose your credentials while your are out of the office??  I would think that you should be able to go in under the administrator account and reset something.  Any help would be greatly appreciated.  Thank you very much.

Doug