My computer started kicking into this mode after walking away leaving it running for an hour or two:

Fans racing, screen saver frozen in motion, noises emmanating from screen or computer?. Had to depress power button 10 seconds and then turn on again. Thought it was just a sleep mode glitch but after the 3rd or 4th time, the computer would not boot up again.

Black screen, no beeps, yellow light goes on for a second when you press the power button. All three fans run constantly. Monitor works with other computer.

Have checked PSU 12V and 5V rail and they're fine. What other checks can I do?

Don't want to be stupid by running out to buy a motherboard or other part without knowing for sure what the problem is.

Thank you