This occurs mostly due to bugs in the IOS software, but could have it's root in a hardware-failure as well.
See this link from TAC about troubleshooting Router crashes:
http://www.ciscotaccc.com/core/showcase?case=K66469183
They refer to another link about troubleshooting as well:
http://www.cisco.com/en/US/products/hw/iad/ps397/products_tech_note09186a00800b4447.shtml
But anyways, with the info you have supplied, it's impossible to say what was the cause. I suggest you contact your support-organization (where you have your technical support-agreement for the equipment) to get a resolve for this problem.
