Clips uploaded recently (less than a month) do not play.

I have been creating a project recently and, when I had to re-import all the videos (keeping the duplicates), all the videos in the project refuse to play and all the thumbnails are gray. I have been able to play one, out of around 50, clip and have gotten the sound on another. I need to finish this project in a month and there is still quite a bit of work to do, so I need this fixed soon.