I had this several times, my Galaxy S3 freezes and then the screen goes black and I can't wake it up. the only way is to reboot it and pull out the battery.
What could be the reason for it?
I also have that problem. It starts when the battery goes really low.
So, keep it charged up. If you take the battery out for awhile, it will reset.
But, get another battery. Best Buy has them for as low as 8 bucks online.