Check your internet connection. A poor or unstable connection can cause organizers to temporarily not load.

Check that iCloud is enabled for Notability. Go to Settings > [your name] > iCloud and make sure Notability is toggled on.

Force-quit and relaunch Notability. Swipe up from the App Switcher to fully close Notability, then reopen it. This step may need to be repeated a few times before Folders rebuild.

Check all your devices. If you use Notability on multiple devices, check whether Folders are visible on another device. iCloud sync can sometimes take time to sync across devices, so your library may appear correctly on one device before others catch up.