Samsung Galaxy s3 only works when shaking?

I got my Samsung Galaxy s3 last year about this time and I have had no troubles until a few days ago it rebooted itself, and i figured i had pushed the power button in my pocket or something. But today, i went to plug in my phone and when i looked again, it was off. I have tried taking the battery out and putting it back in and restarting the phone, but for some reason it will only turn back on when the phone is in movement? I have no idea what to do about this or why its happening...