I know this is an old story, but I haven't seen this exact problem. My S3 will shut off probably once a day by itself. It will not turn back on unless I remove the battery for a couple of minutes. Then it will start up and run fine. Anyone else experience this particular issue? I know there are other posts of the unit randomly turning itself off, but I haven't seen anything about removing the battery to get it going again. No rooting or such on the phone...on Android 4.3. Was thinking of updating to KitKat, but now thinking twice about that til I figure out what's going on with the shut-off.