Why does my phone keep shutting off completely

So my phone keeps going to the restart white screen, and cycles from that to black screen. It happened a week ago out of nowhere, but then the other night I sat on it on my bed then it went to the restart screen after it being completely died for a week and ended up turning back on. It now though will randomly go through that whole process of shutting of but if I sit on it for a bit it’ll go back to white screen and turn back on. So, what is probably broken and how should I fix it?