What watt power supply? and what runs off it? It may be overloaded. Does the fan work? Replacing it may be easiest, and is less than $40. Otherwise, use a multimeter to check the voltage level at each supply output against a pinout chart for your power supply.
(note-2)regarding power connections for the 20-pin plug; ground is shown as connector numbers 3, 13, 5, 7, 15, 16, 17; and the PS ON is number 14 (good luck, wear rubber sole shoes (ha) )
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ATX Power Supply