I still have access to the disc, but I lost the piece of paper with the key. Can I somehow recover it? I already had a look into Keychain Access Utility, but I couldn’t find it.
What would be the purpose of disk encryption if the key can get recovered from the encrypted disk itself? Unless you find it in the Keychain (and I'm not even sure it's there in the first place) or somewhere else where you may have written it down, you are out of luck.