I was playing around with the Find My Mac iPhone app (just testing it out) and hit the "lock" option. My MacBook Pro is now locked and I have no place to enter the 4 digit PIN.
When I start it, it shows the Apple logo with a loading circle and it shuts down. If I start whilst holding alt it asks for the PIN, which I enter, then it shows three partitions: Macintosh HD, Recovery & Windows. If I select Windows it works perfectly, but the two others show the problem with the Apple logo and loading circle before it shuts down.
Best Answer
Boot into internet recovery. Hold down option + command + R at the chime.
Use Disk Utility to repair the volume and then reboot your machine normally. You should be able to unlock it now.