Do not worry, we suppose your collection is fine. Be sure, that you authorized with the right account.
To check this, open your profile and find your ID on the top of the screen.
If you are authorized in the app with the account you'd used before, just close the app (press the "Home" button on the bottom of the screen twice and swipe up the miniature of the app) and then open the app again. Afterward, check your collections. All tracks, albums, and playlists must be at their places.
If you are not authorized in the app, just press "Log in" in the "Profile" menu.
In a case this recommendation didn't help, contact us by using a feedback feature in your profile's "Help and Support" menu inside the app.