Try first by clearing the pram, restart/boot up and hold Command + Option + P + R until it reboots again by its self.
If that fails: make sure you've got wifi, restart/boot up and hold down Command + R, select reinstall (or format and reinstall) and follow the instructions.
Failing all of that, book it into a genius bar at an apple store and they'll be able to point you in the right direction.