HI, I'm hoping someone can help me. My MacBook has suddenly decided not to start up, at all. It has not suffered any physical damage. It was working fine about two hours ago, and now it will not start, by battery alone or by charger. I tried switching the battery with an extra, new battery, but that had no effect. I thought maybe I could start up from the system disc, but it won't even slide in and stay. I would appreciate any help with this. Thanks!