Were there any changes to the ADS or Kerberos? Check the following:

Was there an IP Address change on the NAS Server?

Try deleting the computer name in your Windows server then re-enter the NAS server computer name again w/password - Then enter the Domain / administrator & password in the NAS server again.

Check your DNS setting Forward and Reverse for the NAS server.

Check any changes to WINS servers.

If you are using any Kerberos please provide the errors from the Windows server "Event ID errors".

Try going to PDC if this connects then go to ADS authentication.