This has happened to my countless times and it is so annoying. Fortunately, it is easily fixed.
Reboot the Mac by pressing the power button to turn on like normal. As soon as you hear the boot chime, hold down the command, option, P and R keys all together at once and keep holding.
The mac should then reboot and make the boot chime again, whilst displaying the Apple logo on the screen as in a normal start-up. All should be like normal from here.
I hope this helps!