My first guess would be a faulty Power Supply Unit, if it switches off just like that, although that wouldn't explain the not switching itself off when freezing.

You really need a 2nd PC here (even old would do) to test all the components in a working box, but mainly test the RAM, Video Card, and CPU (invidually, try at the same time in the other PC afterwards if they work on their own with working components in the other machine) since that's what you need minimum to be able to see something on the screen. Best to leave HDs disconnected, since they won't affect POST from beginning to display, and you don't really want to corrupt them

Or, if you had the other PC, swap the PSU units around to see if that's the cause (might be messy due to the huge amount of wires coming out of the thing).


Other thing you could try (either before or after testing the PSU) is to reset the BIOS configuration to default/safe - you'll probably need your motherboard manual to see which jumpers you need to set on the board to clear/reset the CMOS - since it's possible something is borked in there and is doing something invalid.