If you see a warning upon opening the app stating "Preparing your library for a faster experience.." generally speaking the issue should resolve itself eventually. However, depending on the size of your library, it might take some time for the app to dismiss the message. Please leave it visible on the screen even if it seems stuck.
If the process hangs forever or seems stuck and you have a proper backup (i.e iCloud / auto backup or manual backup), you can uninstall the app (this will remove all your local app data), reinstall and restore the data from your backup.
If the issue persists, your library might be corrupted and should be sent to us for recovery.