Power button on Dell Inspiron 518 Desktop does not work

I have a Dell Inspiron 518. The computer worked fine for a long tme, then the next day i pushed the power button and nothing happens. i tried using another working power cable but it did not power up. The only light that comes on is a yellowish green light on the back of the tower just below where the power cable gets plugged in. I took the front off and used my finger to push the button on the inside but it doesnt turn on. Any help is very much appreciated. Thank you.