If the monitor isn't working, how do you know keyboard and mouse don't work?
Sounds like a video output issue if monitor works on another PC. (Was that tested using the same video cable from the monitor to the other PC?)
Is monitor connected to onboard VGA port or to an add-in video card? If it's an add-in video card, try reseating that card in its slot on the motherboard. If that doesn't help, remove that card and try connecting the monitor to the onboard VGA port.
NOTE: Always power off, unplug and press/hold power buttonfor ~15 sec before doing any work inside the case.