I've had this happen a few times . You can use the recovery mode to bring it back to original . On a couple occasions what worked for me was to go into recovery mode then cancel the reinstall at the prompt stating everything will be wiped out (or something to that effect) . That was enough to get it working again , I'm not sure why or how it tripped it to work again but it did . On a couple other occasions that didn't work and I had to go through the full recovery which wiped out my settings , programs and all . I've tried numerous times to get to the os and back up when this has happened but that was always a fail