PS4 turns off by itself/crashes

For the record let me just say I know precious little about the PS4, I have a past as a computer technician but it’s been a hot minute. Here lies my issue:

I had an ex boyfriend leave a PS4 in my apartment when he moved. He said it was overheating but I don’t think that’s the case. I opened it, cleaned out all the dust and re-thermal pasted the CPU. Put it back together. It turned on just fine, and did the needed updates, download some stuff, etc. Turned it off so I could go to bed and work early this morning…all seemed well.

This evening I came home, I did a few things like put a passcode on it to keep my homeschooled kid out of it all day. While trying to open a game for the first time it just crashed. When it restarted it gave a notification like “Checking system storage” or something similar. To me, the verbiage sounded like a hard drive issue, so I pulled the hard drive and replaced it with another unused one that I had, got the files from PS’s website and loaded everything onto a jump drive.

It took the reload without incident, along with updates, but when I tried to start up a game (nothing too taxing either, just Minesweeper) it took a hefty shit and the screen went black with “No Signal”. The screen went fully pixelated with a multitude of colors shortly after that, so I’m not sure what was going on.

I’m really confused as to why this happened and I’m even more stumped as to what to do to remedy the situation. I’m hoping maybe someone here has had a similar issue and knew how to fix it?