Hash functions are generally irreversible so if you expect that you can apply some "inverse hash" function on hashed passwords to gain the normal/original passwords, it is not possible.
If you want to obtain the key, possible solution is to apply a "brute force" attack (if you are not aware of brute force, it simply means applying all possible keys). You'll have to use some software for this because you can't manually try this. You need to know at least the size/length of the old CryptoKey so that you can try all possible combinations for that key size.
Another option is to analyze the available hashed passwords by applying the frequency analysis techniques or dictionary attacks. This may work if you have not added any salt to the key while taking hash.