Sponge also takes a backup of the level file on each save of the world. You’ll find it in the same root as the level_sponge.dat file and is called level_sponge.dat_old. Assuming that file isn’t corrupt, do as @simon816 said and backup your current data but, instead, do the following:
Rename level_sponge.dat to level_sponge.dat_bad
Rename level_sponge.dat_old to level_sponge.dat
If this is still corrupt then you’ll need to delete these files and the world implementation will re-create them on first save. Keep in mind this WILL reset most world attributes.