If you hide a photo from an Album or Collection, when you click into either Collections or Albums in the Aura App you'll see a small gray tab at the bottom titled "Hidden." This is where you'll locate any of the photos you've actively hidden and will see the option to unhide the photos.
If you hide a photo from your "Frames" photos that previously existed in your Photos (Camera Roll), the photo will be restored to your Photos and can simply be re-added to your frame. Unfortunately, if you hide a photo that was contributed to your frame by somebody else there is no way to retrieve it unless you ask that person to re-add the photo to your frame.
Some photos are hidden automatically thanks to Aura's built-in quality filters. Aura will filter out IDs, screenshots, panoramas, and other photos that you wouldn't want to see on the frame. You can always un-hide these photos by following the steps above.
If you require further assistance please don't hesitate to contact Aura's Customer Support by emailing email@example.com or calling 646-630-7237.