Unable to power up computer

I have a Dell Optiplex GX260 that will not power up. It was working fine one day, powered off for the night and the next day it does not power on. No disk or fan sounds. Power light is amber and the light on the mother board is lit. No beeps, no LED lights on the back. I’ve tried a good power source. The PC had not been giving any problems until it just decided to quit. I’ve reseated the cables and memory. How can I tell if it is the power switch, the power supply, the mother board or something else? Thanks in advance.