Could be a problem with the power switch. To check it out, remove the bottom panel, disconnect the battery (always disconnect battery when working inside the machine), disconnect the keyboard cable, find the two power pads, short them with a small flat screwdriver (charger connected), fan should start rolling and you'll hear the startup chime. If the machine function normally without restarting then the keyboard (power switch) is faulty and has to be replaced.
Could be a problem with the power switch. To check it out, remove the bottom panel, disconnect the battery (always disconnect battery when working inside the machine), disconnect the keyboard cable, find the two power pads, short them with a small flat screwdriver (charger connected), fan should start rolling and you'll hear the startup chime. If the machine function normally without restarting then the keyboard (power switch) is faulty and has to be replaced.