My 2012 MacBook Pro Mavericks has suddenly stopped turning on properly. I get the start up chime and the keyboard lights up, but the screen doesn't turn on, it remains black. I always just close the lid to turn it off, but occasionally i do a full turn off with the power button. When I used the MacBook earlier today I turned it off by holding the power button down. 30 mins later I turned it back on and started experiencing the problem. I've made a Genius appointment for next Friday, hopefully my 3 yr Apple Care will save me, but is there anything I can try myself in the meantime please?