I assume you mean it turns off and then back on.
Disconnect the disk and connect a monitor.
What does the monitor show?
This PC has been running fine and strong for a long time, I turned it off the other night, and now Thisbe morning trying to turn it on resulted in a loop of my PC trying to boot and then shutting down.
I think was working fine the night before.
The video is the best example, it even stays on for a good 30 seconds before failing not just a quick off and on.
Any help is seriously appreciated!