Reseat the memory modules (remove and reinstall them) and verify that the LED on the AC adapter is solidly ON.  If it's not, replace the adapter.

If the above does nothing, it is almost certainly a mainboard fault.

Hi,   I tried reseating the memory,  the AC adaptor is fine.  tried 2 different ones.   It will turn on I found out if I press on the left side of the power button (where the caps lock, num lock lights are) almost everytime.   very wierd......