Hopper8's 'Odin troubleshooting' thread stickied in General, 3 part firmware linked to in thread, download, flash with Odin (this will wipe your phone). Failure to flash ? Go through the troubleshooting steps in the thread over & over & over (if necessary; all of them, including multiple PC's, if you don't fix it with the info in that thread, you won't be fixing it).
There's no easy fix here; if it's recoverable (and most of these are), they're fixed with perseverance. Those which aren't recoverable are NAND corrupted & require a motherboard replacement to fix.
Edit - Also, I presume we're talking about an I9100 ? If you have any other model, do not flash that 3 part firmware I referred to above, you will brick your phone if you have another device & you flash it.
If it's important enough to cause you grief if it's lost, back it up. No excuses
Trolling for thanks is lame
Does Samsung's auto-brightness implementation crap drive you to self-harm ? A solution !
Failure to search is the leading cause of male impotence on XDA
If you don't know how to recover if things go wrong, what are you doing flashing custom roms ?