Sorry, if your server is gone (e.g., terminated EC2 instance) nobody can help you. Next time either save an escrow key somewhere[s] safe or don't set a passphrase.
Don't misunderstand, we'd love to help if we could, but we can't. The encryption key for your backup was generated locally on your server not ours. We designed passphrase protection to use special cryptographic countermeasures to make typical cracking techniques (e.g., dictionary attacks) very difficult even for someone with access to massive amounts of computer resources.
Note, if the system you backed up is still available, just log into it as root and change the passphrase (you don't need to know the old passphrase):