can a PSU fry/ruin a mb or other components its connected to?

got a used psu. wanted to have it as a spare. guy said fine and tested.

took out my oc. 24pin 8pin cpu and 8pin pci. connected his with the 3 connectors.

not posting. I have 4 fans (3 intake fron of case, 1 exhuast) and all 3 front always spin. rear exhaust spins with a delay, then turns and spins. it could be too many fans on 1 mb connector. but ive been running like this for a year now.

changed to test used psu. all 3 front fans turn but rear is partly turning or trying to but doesnt, try and doesnt

but nothing posting

changed back to my psu and its not posting.

can a psu fry a mb/components?

I experienced a similar case about ten years ago.
(indeed I did similar, then gave the PSU to my friend, later I re-bought an Antec PSU).

new motherboard has better protection circuits, shouldn't have this problem any more.

Yes a bad PSU can damage things, usually due to being a low end PSU without sufficient shutdown protection.

I'd also wonder if during the PSU swapping, an ESD event scrambled the bios, would at least try removing the battery and using clear CMOS jumper/switch/solder-pads, whichever it has, replace battery, try again. Some boards (rare but not unheard of) will also not post if their battery is dead, something you might not notice until PSU is removed so not getting 5VSB power. Some boards now have a dual bios or other recovery mode using a USB flash drive to blind reflash the bios - check the mobo manual.

I assume you did not overlook anything, like there was one time I forgot to plug in the CPU 12V connector.