xen1:~ # grep boot /var/log/messages
Jan 20 19:41:12 xen1 shutdown: shutting down for system reboot
Check to see if the system was manually rebooted by using the last facility. +(Hint: Look for the user "reboot".)+
Otherwise, assuming that your system has hardware logging, I recommend checking the system BIOS system log to see if there were any hardware related failures (e.g., CPU temperature, power supply anomalies, etc).