The first time you backed up after encryption was enabled you were prompted to enter a password - twice for confirmation. If you did not check encrypt backup, but you had a company email account on the phone, the company Exchange administrator had the option of requiring an encrypted backup. But you would still have had to create the backup password. The password first entered will not be prompted for again when you back up, it will reuse that same password, even if it was years ago that the first encrypted backup was made. You cannot restore the phone without that password, however, as sberman posted a link, there are instructions on how to make a new backup with a different password (or no password). If you enabled Keychain when the password was created you had the option of saving the password in the keychain app, so check there first.
You can try guessing the password; you get unlimited guesses. To speed up the process click to change the password or uncheck encrypt backup and you will be prompted immediately. If you are like most people it is a password that you reused; so think about what you may have used, such as your Apple ID password, your phone unlock code or your computer account password.