Check your last login logs etc, make sure nobody has control of your box.
If a linux box crashes it normally doesn't come back again, so I'm thinking this is more likely hardware, bad RAM CPU or PSU possibly, all of which you'll really need to take the machine offline to fix.
Then again if it were a hardware fault your reboots would probably be hard reboots, and you'd see that logged.
█ Dan Kitchen | Technical Director | Razorblue
█ ddi: (+44) (0)1748 900 680 | e: [email protected]
█ UK Intensive Managed Hosting, Clusters and Colocation.
█ HP Servers, Cisco/Juniper Powered BGP Network (AS15692).