If you noticed exclamation marks appear on your previous creations or on the content you uploaded, this may be due to the removal of the related media from your camera roll.
As the app is currently not storing content locally, your photos and videos should always be saved on your camera roll to be used to create and reproduce projects with the app. Please, don't hesitate to try and re-import such media in your camera roll: this should allow your projects to be visible and editable again in Background Music.
If you deleted the file recently, you might be able to recover it from the Recently Deleted folder on your device. If you backed up the file to a different location (such as iCloud), you can download it onto your phone once again, then add it back onto your project.