Thank you for using OnSong!
OnSong automatically archives older sets after they hit a certain age. This helps your library from getting cluttered. When a set is archived, it is automatically moved to the Archived tab at the bottom of the sets menu.
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 in edit 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 all together, go into the Utilities menu (gear icon) --> Settings --> Menu Settings > Songs > Sets > Auto-Archive Sets > Never. You can then unarchive the sets you need and OnSong will not automatically archive your sets.
Hope that helps!