Q: Quicktime Player won't play videos in Photos

Videos that use to play in iphotos will not play in Photos now. I see the video thumbnails there, but when I click on them I get an error message. Quicktime X won't covert these videos and I'm at a loss as to what I need to do? I thought I could recover these videos from my backup hard drive but no luck. I've read some other people may have had the same issue but not sure what the best solution is to get these videos to play? Do I need to really go to a 3rd party video converter? Thanks for any suggestions.