If the value has been ENCRYPTED and the key known, it can be decrypted. Obviously.
If the value has been HASHED, then that's another tale. Hashing is supposed to be irreversible. However, I believe someone, somewhere, has broken the SHA-0 hashing (just a rumour I heard.) SHA-1, according to Wikipedia, has theoretical attacks. SHA-2 and SHA-3 are, to my knowledge, not breakable with current technology.