PC restarts itself repeatedly.

Here is the pcpartpicker list of my build: https://uk.pcpartpicker.com/list/zVby8M

I know very little about computers which is why I came here (literally building it took me like 8 hours with help).

I’ve managed to work out (I was told) that it wasn’t a Windows problem, because I do not get any warning that the system is restarting, it just flicks straight to a black screen and restarts itself, which obviously means it’s most likely a hardware problem of some kind.

What could be causing it, and how do I fix it?