I suppose your xbox is softmoded.
2 ways, you choose:
1.- with xboxHDM.
you will need your eeprom.bin(which you should have backed up when you soft mod the first time)
if you don't have your eeprom go
HERE.
For xboxHDM just read
HERE2.- Get a modchip(buy it or borrow it)
install the mod chip and depending on the chip, the chip OS can lock it, or get an auto istaller(Slayers) and lock it