My server did a reboot today in the middle of the day, where should I look to see what may have caused this to happen?
It is a Windows Server 2008 machine
Definitely check the event log, but also depending on what brand of server it is, it may have its own internal logging system which is well worth checking.
For example, my HP servers have an inbuilt logging system that once told me a server rebooted in the middle of the night, and immediately before it did that there was a hard disk went bad and the RAID controller just was not happy and blue screened Windows. All I got from the Windows event log was "The last shutdown at 02:28 was unexpected".
The Event Log - particularly the System log around the time the reboot happened.
You might find it was a BSOD (blue screen of death) - more information should have been logged to the event log.
I ran across a tool called "WhoCrashed" located at http://www.resplendence.com/downloads.
From the website:
Comprehensible crash dump analysis tool for Windows 7/Vista/XP/2008/2003/2000 (x86 and x64)
This thing looks at your crash dump file to determine what might have caused the crash.