You may have suffered a power blip in the night that would have powered off your pi Unless you had it on a UPS.
Once you apply power to a RPI it powers up. No switches here. The HG service daemon is set for restart=always so a power outage and reconnection wouldn't be a reason for RPI not starting and HG not launching. In some rare cases you may get file corruption that could prevent it starting.
It's understanding the basics of server technology that helps troubleshooting problems. The tools are there to be used to do this. Firstly, a simple Putty into the Pi to see if it's accessible. If so a sudo systemctl status homegenie.service to see if HG is running.
I think the lack of basic server knowledge in relation to how HG functions would be at the root of the users problem. I've given him two pointers as to what to check for but this information is all over the web for anyone to see.