Windows event viewer application logs lost after reboot

Open event viewer. Expand windows logs. Click system, then either find or filter for event ID 1074. And you will see all your shut down logs.

The first item I would check is the HP Integrated Management log. Assuming that all of the ProLiant SupportPack is installed, you can view the log by using the following url when logged on to the server: https://127.0.0.1:2381 , and select Logs from the top menu. If there is a hardware problem, that will appear as a yellow or red splat on the main page.

Another option is if Windows is still alive, you can enable a feature that will force the system to generate a memory dump before restarting. You will need to have a pagefile that is the same size or larger than physical memory for this to work. When the symptom occurs, you can press to force a blue screen crash.

Forcing a System Crash from the Keyboard
http://msdn.microsoft.com/en-us/library/ff545499%28v=vs.85%29.aspx