I see this all the time at work. Here are the basic things to do first.
Is the user logged into any other computer?
-Did they select switch user instead of logging of a computer one day?
as suggested by cgc018 (this is a great tool)
-Have your user shut down any computer they are logged into
-Unlock the users account either with AD or LockOutstatus
-Keep an eye out on Lockoutstatus for Bad Password Attempts (give it a good amount of time) you will have to press f5 to refresh.
If the bad password attempt goes up it is an issue with something other than the PC (As the PC should be off)
-Do other systems use the same password as the LAN password? For example
--Any Syncs with any different programs?
--Any syncs with any web portals?
--Are you sure they are logged of another computer
If the bad password counts do go up its pretty safe to say its from the machine end. One of the main things to check from here is inside the Windows Credential Manager and just delete any stored passwords.
Have you tried a full profile reset?
This may seem like its asking more questions that Answering yours, but its just a case of pin pointing the cause.