Green power light, turns off after 3 seconds with no warning

Hi, I have a PS3 Slim that I just wanted to play Gran Turismo on after a long time. Plugged it in normally, red power light is on, so I obviously pressed it.

You get the beep, the light for the eject and power buttons, as well as the green light above the power button (but no blue light for the disc drive even though I have a CD in). After about a second the green light disappears, although the lights for the buttons are still on, until about 3 seconds later where the entire system just turns off. All without any other sounds.

Obviously when you press the button again, the red LED comes back on, and when you press it after that it’s the same all over again.

I have absolutely zero experience with game consoles, other than a PSP1000 that had the reliability of a 2009 BMW E90 - which I ended up taking a hammer to, obviously from frustration. As I currently don’t have the proper screwdriver I can’t open it, but is there anyone who knows what could be wrong with the system?

Thanks.