SymptomsSometimes server is restarted for no visible reason. Inspecting event log you can see the error:
The system process 'C:\WINDOWS\system32\lsass.exe' terminated unexpectedly with status code 0. The system will now shut down and restart.
CauseMost likely the server is infected by a virus.
ResolutionRead Microsoft Articles regarding this problem:
The best solution recommended by Windows manufacturer is to perform a complete "clean" installation on a previously compromised computer to remove any undiscovered exploits that can lead to a future compromise.