The first thing is to see if you feel the fan on the Power Supply unit is turning when the computer is on. If not then replace the supply unit. The second thing is to open the computer case and see if the 4 plug Molex Connector is properly plugged into the hard drive units. If so, then unplug it and get a multimeter, set it for VOLTS DC and with the black lead of the multimeter plugged into one of the black leads of the Molex connector measure the red and yellow leads. The red lead should measure -5 volts and the yellow +12 volts. If either voltage is missing, replace the power supply.
