My PC turns on then off?

There's actually quite a few things that could be causing the issue and it's really hard to properly answer your question without a lot more information. For example at what stage does it reset and go into the bootloop? Does the computer post successfully?, or does nothing happen? Does it attempt to boot into the OS, or does it die before it even tries? Does the computer beep, or indicate an error message in anyway? Do you hear a clicking noise when powering up the computer? Some OEMs even try to indicate error codes through the power LED, do you notice any unusual activity with the LED on powerup?

Anyway, to get you started the first things you want to do is to perform a CMOS clear. Turn off the power from the PSU, if you're unsure unplug the power source from the back of the computer. let it sit for a bit. You can also insure that all power has drained from the circuit by pressing the power button while it's unplugged/ turned off from the PSU switch. After that open the case and pull the small batter out of the main board. It's a small silver battery like you would find in a watch. Let it sit for a while then go ahead and put it back in. Close the case reconnect the power source and turn on the power switch on the PSU. While doing this make sure you're touching the aluminum/ metal of the computer case to prevent static discharge from arching to the main board in the computer.

After having completed clearing it, if it still doesn't work move on to testing your memory. Remove all sticks of memory and test them individually. While you're in there check that all wires/cables are properly secured to ensure you don't have a bad connection that could be causing the intermittent behavior.

Sorry I can't help you more than giving you someplace to start without more information. Good luck.