Start up light flashes once and it won't power on
Most probable cause is the power supply. Look at the back of the computer, check the voltage settings located near the power cord. This will be the small red tab indicating two different settings, If your in the US it should be set to 115. Also, the power supply should run the internal fan when it is turned on.
To test the power supply try this:
Open your computer case and locate the gray box that is the power supply unit. Trace
the wires from the power supply to the boards and devices and
disconnect all the cables by unplugging them.
Find the big connector plugged on the the motherboard, make sure it is disconnected and short the two pins: (GREEN & BLACK) Short green pin (14) to GND (any black) to switch power supply ON, disconnect green from GND to switch OFF.
This will switch the power supply, you should hear or see the fan in the power supply spin. If you do not see the fan spinning, and your sure you have shorted the correct wires, you have a faulty power supply. If you see the fan spinning, you can now test the voltage coming out of the power supply.
Place a volt meter on the following colors to test voltage:
The color code for the wires is: Red = +5V, Black = Ground (0V), White =
-5V, Yellow = +12V, Blue = -12V, Orange = +3.3V, Purple = +5V Standby
(not used), Gray = power is on (output), and Green = Turn DC on (input).