SAP instance unexpected termination

hi sap experts

i am running erp and crm on one server, few times our all sap instances terminated with out any prior warning, if i restart working fine, but i want to know the reason why they terminates and what i should do to prevent it happen again. i see the log in event veiwer as "EVENT_SERVICE_CRASH_NO_ACTION".pls help

thanks and regards

sandeep