Computer turn on and off itself? Help

Hellow guys, i'm also currently have an issues where, when i switch on the main switch, the computer turn on itself, i didnt even press the power button! And after a few second, maybe 2 to 3 or 4, it turn off itself again. When i press the power button myself, there is no more power. I cant turn it on.

So, i do the same thing again. This time I switch off the main switch and turn it back on and the same thing happen again, the computer turn on and off again, and i also cannot switch on the computer after that. The same thing happen the next few time, so i assume there is something wrong with the hardware, or something loose? Any idea anyone?

And, to be more specific, when i turn off the main switch and turn on immediatly, there is no power and the computer is not running. But if i wait for maybe a few minute after turn off the main switch and turn back on, it run itself, the same process as i mention above. So is it weird? Hardware problem?

did it just start doing this out of the blue? Did you do something before this started?

The first thing I'd check is the power button on the case, as you say it does not work and if it's jammed on the ON it could give similar issues.
Open the case and remove the power button connector from the motherboard's pins, and if you can install in its place the connector of the reset button. Both buttons are the same under the hood so it's not going to hurt the board.
Now try using the reset button as if it was the power on/off button.

Yup, it just happen that way, i didnt do anything. Yesterday the computer still working fine, but today when i switch on it just happen to be this way. Didnt do anything else before this happen.

The power button at the CPU case is still can still can press, which i think nothing wrong with it, but i did try to unplug and connect the wire that connect the power button from the motherboard. I disconnect it from the board pin and reconnect. Nothing happen. But i havent try the way you suggest, i'll give it a try soon. Wait for my reply :)
Now i have exam, cant focus much on computer, later :)