if I move my mac book air, the screen goes black and I have to do a hard reset

If I move my Macbook Air, the screen goes black. Originally I noticed it if I carried it with the screen open, as if to go show someone something. This weekend I traveled with it for the first time (closed, of course and shut down). When I got ready to start it, it wouldn't start - I had to plug it in and do a hard reset.... Any ideas?