Hold the power button for 10 seconds to shut it down, then:
2. Plug in the power adapter to a power source, connecting it to the Mac if its not already connected.
3. On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time, if it starts up you held the power button before the other keys, try again.
4. Release all the keys and the power button at the same time.
5. Press the power button to turn on the computer.
6. Press and hold the Command-Option-P-R keys before the gray screen appears. (You have to do it pretty fast if you miss it just shut down by holding the power button for 10 seconds then start it up and continue with step 6).
7. Hold the keys down until the computer restarts again at least twice.
8. Release the keys and immediately hold the Shift key until you see the Apple logo. This will activate Safe boot and it will check the drive for errors among other things. It may take longer to boot and you may see a progress bar during part of the process. If it starts up OK, login then restart the normal way from the Apple Menu. Let me know after.