What’s causing my PC to partially turn off upon start up?

First thing in the morning for example. I turn on my computer, it will do its load screen and bios stuff and as I am logging in, it shuts off. Cooler fans still light up and so does the RAM— it’s not fully shut off? Sometimes, it’ll turn off immediately after turning it or sometimes just before the load screen.

I built this myself years ago and only when I moved it started this problem. Maybe something happened because I had it in the trunk of my car standing upright? I’m not too sure what’s caused this.