Hi, I'm assuming you have an A1278 MacBook Pro from 2011 or 2012 with an i5/i7 processor as these laptops start up automatically when connected to power. The issue with the power button can be caused by the logic board or the keyboard, and is usually a fault of the keyboard. To determine the issue, try to start the laptop up by shorting out the power-on pads on the logic board (search : [your macbook model] power on pads to get the location) with a screwdriver or tweezers. This will send the SMC_ONOFF_L signal to ground, telling the logic board to turn on. If this doesn't cause the macbook to turn on, then it could be an issue with the logic board. If this does work, then you will need to replace the keyboard.