Inspiron 560, does not power up

Inspiron 560 suddenly decided not to power up. Went to bed one night after turning the computer off. Next morning it would not come on. I have tried the following tests after reading various posts but have not found  a cause.

1. The light at the back of the power supply is green.

2. Everything is disconnected from the unit.  Every time I tried anything, the power was disconnected and dissipated the static by touching the metal frame and waited till the green light went off.

3. If I connect the power cord, the light in the front power button turns on without me pressing it. All the three fans come on and in just under 4 seconds they all stop but the light at the back of the power supply stays green. All the while nothing inside was touched.

4. I checked the CMOS battery. It is fine showed 3.1 volts sing Fluke.

5. I disconnected USB ports in the front from the motherboard, no change still green light and fans cut off in less than 4 seconds.

6. I disconnected one at a time, the optical drive  and  the hard drive  but no change, with green lights and fans cut off in less than 4 seconds.

7 After I disconnected the power supply to the motherboard, the fans did not run but he green light stayed on.

8. When the power cord is plugged in, after the fans cut off, the light at the back stays green as long as the power cord is plugged in. At this stage if I press the power button in the front of the tower, noting happens.

What can be causing the computer not to turn on and stay on?

Thanks.