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.