[CentOS] Server turns off unexpectedly

A few weeks back I built a new Centos 7 server from a 4 year old workstation PC, and a brand new HDD.

This PC had no prior problems other than it ran Windows 7. Thew new HDD is because I'm planning to also use it as a Bacula storage device.

For the moment is purely an AirPrint server, using CUPS and Avahi so is ide 99.9% of the time.

My priblem is that twice now it has turned itself off, the last time being at 7:40am today. There is nothing I can find in the logs prior to it going off, just the normal start-up entries when it's turned back on.

Can anyone suggest why i may be happening, or how I can diagnose it. The server is in a UPS protected cabinet in locked room