if it is indeed the eeprom its a fairly easy fix.
Get your eeprom (you know the one you backed up when you first modded just like all the tutorials said to do) and embed it into a bios , i use cromwell myself. Than flash that bios to the chip (programmer required or at least another xbox to boot the chip and reflash it with bios containing the embedded eeprom).
Now simply boot up your xbox with the modchip enabled that has the embedded eeprom in the bios. Thats it
Now it will work again.
2 notes to mention.
#1 EVTOOL is a great program to use to embed the eeprom in the bios, it has the option plain as day you cant miss it.
#2 If you google embedding eeprom with cromwell you might just find this one site that already has cromwell bios with an eeprom embedded in it already (just in case you never backed up your eeprom). The eeprom in the bios is a banned one, but it will allow you to get your xbox back up and running. Than you can always use configmagic later to reflash the eeprom to an unbanned one.
If you need anymore help pm me.