Hi!
welcome to Android forums!
If you can get into downloading mode then there might still be hope. You can use a program called Odin to reinstall all of the phones files back to the the way it came out of the box :thumbup:
You will need a computer running windows, a usb cable (to connect the phone) and some patience.
Step one: Download this Odin package:
http://www.mediafire.com/?5l32affsvq7yu5o
And this:
https://dl.dropbox.com/u/54328432/PREVAILodin&ops.zip
Step two: Unzip the both archives into a new folder
Step three: Install SAMSUNG USB Drivers to your computer. These are the drivers for your phone. Install it as you would any other program. (click "next" until stuff does stuff)
32-bit :
http://db.tt/eGN4acLK
64-bit :
http://db.tt/IlIkQ4BV
Step four: Power off the phone, then hold the camera, volume down, and power buttons until it says "Downloading..." in yellow text.
Step five: Connect your phone to your computer with your handy-dandy USB cord. Give it a minute to make sure everything is communicating properly.
Step six: Open Odin Multi_Downloader_v4.43.exe from your folder. (Again, only use the version of Odin included with the zip) You should see a yellow box with "com port mapping" above it. Also make sure that "one package" is NOT checked.
Step seven:
Click OPS, and select SPHM820.ops.
Click Phone and select the PHONE.tar.
Click PDA and select the PDA.tar.
Click Boot and select the BOOT.tar
Click CSC and select the csc.tar
Step eight: Make sure everything is set correctly, and hit Start. Now leave it alone. It will take a couple minutes, you will become concerned and want to mess with it. Resist the urge. Once the phone restarts, and Odin says Reset, the process is complete.
The phone may take a while to fully load the first time, but be patient. It should come back to life