What version is your xbox. If it's 1.6 you need X2 bios 5035 for 1.6, if not, you have to use the one marked "Old"
Get both from xbins, load up flashbios and flash it on there.
The "backup" the guy above was referring to is a feature on the X3 (which I keep getting more and more impressed with) the X2.6 doesn't have it, so hopefully you still have flashbios on a bank.
If you don't know what xbins is, go here:
http://www.xbox-scene.com/articles/xbins.php
Or get that auto tool thingy, I think I saw that on these forums.