Mac Pro 6,1 — no signs of life after replacing I/O board

I picked up a 2013 Mac Pro (MacPro6,1 aka "trashcan") from a pawn shop in March. It worked fine for a while, and then I broke one of the I/O board connectors to the power supply while reässembling it after repasting the processors. A friend set me a known working replacement, but after reassembly, my Mac Pro shows no signs of life when plugged in or when I subsequently press the power button. I've tried resetting the SMC multiple times, to no avail. What, if anything, can I do to fix this?

I have no idea; it was pulled from an e-waste pile. I didn't have the money at the time to buy a known working I/O board from, say, eBay, and don't now, either.

I did not remove the processors from their sockets when repasting them.