Welcome to THG. Before we can assist you, we need to ask you some questions.
Did you build the computer yourself or buy an OEM built machine?
If you built it yourself, please give us a list of all components and model numbers. If you bought an OEM like Dell or HP, give us the exact model number and tell us if you added any upgrades or additions.
Likely causes of most computers crashing under load is a heat issue, bad memory, or a failing power supply. Also driver conflicts from your video card or an overstressed video card can cause your system to crash or restart also.
So list all your specs and the full nature of the restart. Does it show a blue screen first? Does it freeze before restarting? Or does it just restart whenever you load the CPU or GPU heavily?