1. You force it to power off, restart and try again to boot in your session
2. PRAAM reset:
Shut down the computer.
Press the power button.
Before the grey screen appears, press the Command, Option, P, and R keys at the same time.
Hold the keys until your computer restarts and you hear the startup sound a second time.
Release the keys.
3.Remove the backplate, disconnect and reconect the battery and try again (you can use this [guide|12340|guide] from 1 to step 3