My $0.02

Some 主板 has a +5V standby 指示燈 and it should be on if the power supply is OK. Put a jumper on the power switch pins and remove it to simulate a power switch on if you suspect the 啟動開關 is bad. Do you smell anything burning in your case? Test your power supply with a ATX power supply tester if you have one. Disconnect all devices like hard drive, DVD drive and PCI cards but not graphics card, CPU and RAM. Maybe the power supply is overloaded, power supply efficiency drop when they age. Shake your case to make sure there is no loose metal object that shorted your motherboard.