Black square appearing in my photos. How can I restore my original photo?

Such a large missing black block looks like a JPEG (or HEIC) corruption.

This can happen, if a bit flips as a transmission error.

This cannot be repaired. What kind of backups are you keeping on your iPhone or on a computer, if you have one? You need to restore the damaged photo from the last backup you made before the image got damaged.

Your screenshot is showing that the photo is in portrait mode. If the damage is with the depth map, it might help to disable the portrait mode.

Have you checked, if the Setting > Privacy & Security > Sensitive Content is enabled? It could be blocking parts of the image. Receive warnings about sensitive content on iPhone - Apple Support (MT)

It is hard to tell what has been happening. Could you post a screenshot of the photo with the black square?

If the square has appeared after you added adjustments, try to revert the photo to the original.

See: Undo and revert photo edits on iPhone - Apple Support

It happened to me recently too. It wasn’t in the other photos I took but the black square photo said I was using telephoto which must have been to zoom in on the building.