The power button on the laptop and all modern computers isn’t connected directly to the power supply, instead, it’s handled by the some electronics circuit on the motherboard. The electronic circuit decides what to do when you press the power button – usually if the computer is off, this circuit powers on the laptop; usually if the laptop is on, this circuit passes an event to the processor, which causes the operation system to either trigger a shutdown, or ask the user what they want to do. This is why pushing the power button doesn’t just cut the power, though most laptop also have an override mode to hard power-off instead of requesting a shutdown from the operation system, this is usually triggered by holding the power button for 4 seconds. When the laptop won’t turn on, it’s possible for this system to be in the wrong state.
For example, if the circuit for some reason think that the laptop is already on, then pressing the button won’t cause the laptop to start up because the circuit would be trying to do the wrong thing. The only way to reset this circuit is to remove power from it entirely. For laptops, you need to remove both the charger and the battery, and wait 1 minut after removing the power is to allow any capacitors inside the laptop that has any temporary stored up charge to discharge. pressing the power button while there’s no power connected will speed up the discharge of capacitors. Once powered down and fully discharged, reapplying the power allows the laptop to boot up from a completely cleared state. Some time when your laptop is not powering on, you may just have a defective power button.
How to test power button continuity:
Turn on your Multimeter, the red probe in the VΩHz port and the black probe in the COM port. Set the dial to Continuity Test mode (the symbol ). Make sure there is no current running through the power button. Place the red probe on one terminal of power button and the black probe on the other terminal of the power button.
The multimeter screen would displays a value of one if there is no pressure on power button, and when you push the power button The multimeter screen displays a value of zero and the Multimeter beeps, there is Continuity!
If not the power button failty and need to be replace .
ٍVideo How to test Power Button ON/OFF Continuity with a Multimeter