I drove to a clients house and got a Dell Inspiration 580s series desktop running Windows 7. The system looks brand new. I opened the case very clean. Anyways, I plug the power cord into the tower I see lights come on the motherboard but when I hit the power button to turn on the computer it doesn't respond it wont turn on. I opened the case and front of the case (checking for loose wires) none were present.

I don't know if it's a faulty motherboard or power supply. I don't have a motherboard tester or power supply tester here with me. The customer wants the issue fixed. The system is less than 3 years old. I asked the customer if any lighting strikes or anything like that he said no....then I asked him when was the last time he was able to get online and he said March he didn't tell me this yesterday... What do you guys think it is? Mobo, power supply, misconfiguration etc....all help is greatly appreciated

ps; i hit the button it's dead does nothing. I checked the owners manual for that make and model dell looking for possibly a security trick but I need help. all help is greatly appreciated