Hi Rajesh,
If the server is up you can check the CUCM Event Viewer System logs for the moment the server restarted.
If the server does not boot and if you have physical access to the server, you can check the LEDs on the server.
You can search specific information based on your server model, both HP and IBM have hardware troubleshooting tools.
If you bought the CUCM server via Cisco, you can open a TAC Service Request and TAC can contact HP/IBM on your behalf for the hardware troubleshooting.