Because SingASong doesn't require you to sign up, your Library is tied to the account created on that device and app installation. Reinstalling, or moving to a new phone, creates a new account, and the old Library may no longer be reachable.
Your subscription is recoverable — sign in with the same Google account and tap Restore. See What does Restore do?
If you've lost a Library that mattered to you, email support@singasong.app with your platform and roughly when you created the tracks, and we'll see what we can find.
Before you reinstall or change phone: download the tracks you care about from Studio.