The law the witch did not know?

That if someone allowed themselves to be sacrificed on the stone table without resistance in the place of another, then the stone table would break and the laws made at the start of time (not before) would be reversed.

Well something along that line anyway. If you want to check yourself, I think Alsan explains it to Lucy after he resurrects in the film and book of The Lion, the Witch and the Wardrobe. For the actual original laws, you'll need to read The Magician's Nephew, where Narnia is created.