Also, try a safe boot, do a forced shut down (hold power button till it shuts down) Try restarting holding down the shift key, this will take longer be patient, continue to hold the shift until you see the progress bar and gear then release the key. Allow it to finish booting, then go to Apple menu>about this mac>more info>power and check all the nfo, ac power, battery and see if there are any things that look out of norm. Shut down and try a smc reset.
Read the articale and click the appropriate link, battery you can remove, battery you should not remove.
Follow instructions, see if it helps.
Could be a dying drive.(hope not)
All the best