I left the cover off a tower PC and walked away while it installed linux (yes, bad idea). The cover had the cpu fan mounted on it but the CPU also had a fan strapped on it. I don’t know if the CPU-mounted fan was running or if the machine was entirely dependent on the cover-mounted fan directly above the cpu.
Someone junked this PC (asus p5b) simply because it’s old (2007). Seems like a high-end gaming machine designed for overclocking. Strangely, when I returned to the machine to find it powered off, I felt the heatsink and it did not feel warm at all. Power supply is apparently fine because a green LED on the motherboard is lit. When I press the power-on button the HDD LED flashes for a split second but nothing happens.
I thought if CPUs get too hot there is a protection mechanism to force a power off. Was that the case in 2007? The manual mentions this:
CPU TM function [Enabled] Allows you to enable or disable the CPU internal thermal control function. In TM mode, the CPU power consumption is reduced. Configuration options: [Disabled] [Enabled]
Would that setting cause a protective shutdown? I don’t know how that was set.
Also check if the HDMI is plugged in and the monitor is on before doing all this. I know it is a bit condescending but trying not to be. Sometimes it is something simple.
EDIT: Try to check all the cables first and unplug and replug. Also try different outlets and whatnot.