Power light remain OFF

Hi,

I have a Dell Precision T5500 and when the Power ON button is pressed, nothing happens; the power LED is not on at all (no amber, no green

and no blinking).

When I press the power testing button at the back of the power supply unit, the small LED light is green and all CPU and graphic card fans runs.

Can someone guide me to solve this issue?

Earlier, this unit is working and able to install windows. Out of a sudden, I got this problem as described above.

Please help.

Thank you.