Do you use a Mac with iTunes? If so, when you first turn on encrypted backups, you are prompted to store your password in keychain. You then won’t be prompted for it with subsequent backups, but will always be prompted for it when restoring.
So if using a Mac, check your keychain for that password.
While you cannot restore any existing encrypted backup without the password, you can abandon that one, reset to unencrypted backups and make a new backup - see About encrypted backups in iTunes - Apple Support. However note that an unencrypted iTunes backup excludes some data (e.g. health app data) that is stored in an encrypted iTunes backup and iCloud backups.