|That guy is an idiot & has no business working on computers.|
Why did you let him replace the power supply? Did you ask for it or did he do it on his own? The 20-pin vs 24-pin plug is a non-issue...all you have to do is unclip the extra 4-pins & not use them.
The siren means the CPU is overheating, not that it's fried...if it was fried, there would be no siren. He either knocked the heatsink loose when messing around inside the case or unplugged the CPU cooling fan.