I use an S4 so I imagine that this would be the same on the S3. Try holding the power button and the volume lower button at the same time for about 10 seconds which will do a soft reset of the phone (Similar to a Control, Alternate, Delete on a Windows computer) to see if it helps.
The last time I had a failed OS update, it worked like a charm for me. Good luck.
Hi,
I was notified yesterday evening I want to do a system update on my phone which required me to reboot my phone. After installing it completely (it said 100%), it then started to open, however, until now, I still see the Samsung icon (which shows upon opening the phone). I tried to remove the battery this morning, however, it is still not opening until today.

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