ESX 5: How to Power On A Virtual Machine from the Command Line

So lately I have been noticing some strange issues with my home lab. Every so often I find my all my virtuall machines are down, however the ESX box is up and running fine. Checking the logs on the ESX server has not helped me get to the bottom of the issue, rather I can just see the messages that the server has rebooted. So at the very least I know that something is happening to the ESX server causing it to crash.