If there was a password change recently then usually a phone, people don't ever remember to change the password there also. Also go into credentials manager (control panel) and cleat the stuff out. but look at the 4740 security event for the machine name, usually (not always) there's one listed.
For tougher cases you can turn on netlogon logging on the dc (google it, and don't forget to set a size value, 32mb should be fine but i think it's in bytes) and check the log, that can help identify indirect causes, the log is in windows\debug\netlogon.log which is renamed to .bak when it's full.