G4 Start up issues

I recently acquired a PowerMac G4 from a friend. They had a lot of stuff on the computer causing it to be very slow so I took as much off of it as I could and in doing that the mouse stopped working. So to fix the mouse issue I re-installed the OS9 and OSX (upgrade) version that I had bought for my Beige G3 (which I am not thinking that was the wrong thing to do). Once everything was upgraded everything worked great. I turned off the computer and now it will not start back up. It is like it is in sleep mode – when I push in the start up button on the tower it lights up as long as my finger is pressing it but then when I let go the light turns off and nothing happens. I have unplugged the power source and un/re-plugged all the cables in the back and nothing happens… Any solutions?