OnSong automatically archives older sets after they hit a certain age. This helps your library from becoming cluttered. When a set is archived, it is automatically moved to the Archived tab at the bottom of the set menu. While you are prompted about turning this feature on, many users may agree to the prompt without fully understanding what was presented.
You can unarchive a set by tapping on the Archived tab at the bottom, next tap on the Edit button at the top-right corner to place the list into Editing Mode. Tap on the name of the set you wish to unarchive and choose the Unarchive Set option.
If you prefer to turn off auto-archiving altogether, go into the Utilities Menu (gear icon) and open the Settings Screen. Next, tap on the Menu Settings on the left and then Songs on the right. Locate the Sets section and then set the Auto-Archive Sets option to Never. You can then unarchive the sets you need and OnSong will no longer automatically archive your sets.