There are lots of network monitoring apps out there - but most take some serious fetling to get working, 99% of them require a dedicated PC / server rather than run on your desktop.
Check the event logs for 6006/6009 (startup/shutdown) messages. Most software will only check if the device is pingable or not. You need to interogate the event logs to see when it was up/down.
Nagios, better than nigh****chman although harder to setup and its free. Also is fully customisable on what sort of tests you do I.e not just ping, it can test HTTP, HTTPS, DNS, SIP, all ICMP protocol's active directory pretty much everything.