Did you plug or unplug a USB device at the moment of failure? If so, shine a flashlight into the ports and make sure a pin didn't break or bend.
Beyond that, it really does smell like a power supply problem, although to be sure, strip the system down to just the mainboard, CPU, one stick of RAM, keyboard, and video (if onboard video available, use that and pull the GPU as well). All other power cables, data cables, peripheral cards, and USB devices need to be disconnected from the power supply and/or motherboard. If it boots normally, there's probably a short somewhere else in the system. If still same behavior, probably means PSU, with the motherboard being next.
Also check around the motherboard for any capacitors that have obviously failed, after five years an ordinary EOL failure is entirely possible. These would usually give general instability warnings a long time before enforcing an outright lockdown, but it's still something to check.
He who laughs last, laughs first next time.