the computer that seems to just not turn on, i've had that happen to me
it's probably your CPU, check behind it and make sure the voltage isn't switched to like the higher number cause if so you fried your entire mother board
also, i'm sure you have checked this already, but make sure the wire from the monitor into the tower is okay. Cause sometimes the prongs break on the plug and then you're just screwed over.
you can either change the CPU and take the risk that it isn't that or take it to a repair man... it wont cost more than $150, unless you fried everything