You may have a dead CMOS battery. I had an eMachines computer that appeared to be dead; I replaced the CMOS battery, and the computer came back to life.
Also, check the power cable that plugs into the motherboard. Make sure that each wire has the correct voltage. You likely have an ATX power supply (most computers do). Go to the following web site to see some very good diagrams which show the correct voltages for the main power connector (the one that plugs into the motherboard), as well as for the Molex and SATA power connectors:
If all of the voltages show to be correct when you test them, then replace your CMOS battery.