[Tootech]

First thing to do is disconnect the power adapter and remove the battery.

That will make sure that it has been fully shutdown.

Refit the battery, plug in the power adapter and try it. No joy, hold down the power button for at least 4 seconds, let go, try again.

If it still won't start and you know the power adapter is ok and the battery does hold charge then you are looking at something internal.

Is the power socket ok - does it play up when you plug the adapter in - sometimes they go faulty and work intermittently.

The mainboard doesn't house a power supply, all the work is done in the mains adapter. You could do to test the one you have or borrow one just to make sure it is not that at fault.

If it is a mainboard fault I'm afraid it is a specialist job. Stripping them down to that level, replacing a board and putting it back together so that it works and there are no spare parts left over is a challenge for a seasoned techie.