I got S3 GT-I9300 and it requires to restart the device to update the software and when accept that it says will reboot for a while , I leave it for two hours and its keep frozen on Samsung logo , I tried the hard restart with up volume + home button + power button and try the factory setting but it still give me the same result , when press the volume down + home + power , its asked me if I wish to continue and I said yes and its still downloading but it take a while now , what should I do to get my phone working back again .
You may have to re flash the firmware. There is a program called ODIN you can use to do this.
First of all install ODIN and the firmware for the S3 and your country on this site http://www.sammobile.com/firmwares/ (search for GT-I9300). You will be able to find many tutorials online on how to use ODIN but it is relatively simple. Here is a good YouTube tutorial https://www.youtube.com/watch?v=-CANyNo5qNE (It's quite hard to hear him over the music but it should help). He uses the S2 but it is the same process.
Use the written instructions on sammobile (note this is the firmware for Germanu, so don't use this firmware unless you are in fact German) http://www.sammobile.com/firmwares/3/?download=29037.