Dell Desktop 531 does not power up

I bought the desktop about 14 months ago. It worked fine until 2 days ago. I usually leave my desktop in standby mode to faster turn it on when I need to use it. When I was about to turn it on 2 days ago, I found that the computer was actually off (not on standby mode). I pressed the power button to turn it on. I heard some noise and the LED light flashed (all usual like when it usually starts) but it lasted for only a second then the computer turned off. I pressed the power button again, same thing happened. If I tried to press to power button several times, the system eventually powered up. But when I turned off or put it on standby, the symptoms repeat. I have already ran the hardware test of Dell (from boot menu), nothing seems abnormal (the computer passed all the test).

Does anyone know what problems my computer might have? Any suggestions/advices are greatly appreciated.

nlhma