First off with keeping it simple, did the rocker switch on back of power supply get accidentally bumped to off position. *Yes this happens and it has happened to me before with a switch that is always in on position accidentally getting bumped to off position after messing with the computer.
Others who have had this issue found that its usually a missing cable that needs to be plugged in like 12V CPU power 4, 6, or 8 pin molex or that the board was damaged in the process of removal or installation. *** As well as front panel connections can be placed onto the wrong pins making it act dead because the push button to boot the system is on the wrong pins like the reset and power switch pins the connections getting swapped or staggered across wrong neighboring pins.
Seen boards dead because screws that mount the board are overtightened cracking and crushing the board layers, a slipped screw driver can wipe out a trace killing a critical signal that the board cant function without. As well as if the board came out hard and was flexed it can break internal traces or open up cold solder joints that were connected barely prior. *I'd also unplug and replug all connections to make sure they are good connections and inspect the pins in the connectors before plugging them back in for damage.
Power supply failing during this process is just about unheard of. It could happen but its more likely that the board is wired wrong from front panel connections as my first guess, a damaged pin or cable still missing like 12V CPU power as second guess, or the board was damaged in the process as my 3rd guess as to whats wrong.