my halo 3 disc is EVIL

the disc is evil i tell you so here is how it hapend i am playing the level cortana on legendry i have just goten to the reactor room but i have to go do sompthing elso so i save and turn the 360 off and then later i come back turn it on and go to the level again then the whole system throws a trantrum and all my progress on that level got deleted and i hate HATE that level the enmies and the enviroment has this hapend to aneyone else because this is horible i mean the film of me playing is still there and it shows an hour and a half of me just trying to get through that room with the pelican