This excellent network monitoring tool can use SNMP to retrieve how log a system has been up, and generate alerts if this value is too small - that is, something has crashed or restarted unexpectedly. The reply is in msec which is fine, but humans prefer days and hours, so use this string in the 'Tune up reply value' box to do just that.
[(%SuggestedReply% div 100) div 86400]days [((%SuggestedReply% div 100) mod 86400) div 3600]h [((%SuggestedReply% div 100) mod 3600) div 60]m