When you originally installed OnSong, those purchases were made with your Apple ID. This is an Apple-managed account that allows you to purchase apps as well as Apple products and services. However, since its launch, OnSong never had an account system. You may have registered your OnSong purchase which allows us to stay in contact about important changes with OnSong. No accounts were created during that process.
Since the introduction of OnSong Premium in December 2019, accounts within OnSong are optional but are required if you want to be an OnSong Premium member. In November 2020, we release a new and free version of OnSong. This version requires an account to be created and you can optionally subscribe to be an Essentials or Premium member.
If you are being prompted to sign in and are having trouble, it's likely that you have never created an OnSong account in the first place. We recommend creating an account. Remember that if you are an OnSong Pro user, you continue to maintain your Pro-level features and you should always install the OnSong Pro version of the app from the App Store.
If you have created an account and are still having trouble signing in, you can learn more here.