Quickly! My desktop is a blank black screen (icons are still there), how do I put the picture back?

It’s my parent’s computer, and if they see that the desktop is screwed up, I will most certainly be blamed. I’ve tried setting the picture back, and it actually says that it’s already selected, it just won’t show up. Any idea what wrong? I had turned on high contrast earlier, and then promptly back off, so I don’t think that would be a problem… I just can’t figure it out.