Video files now not working

I had entered the video files into my project yesterday and they were playing fine. I have opened up the project today and some of the video files have errors, but when I play the project even the videos that have thumbnails come up with an error that they can't be played. Even removing and adding again is getting the same error (0x87160139). The files play fine in Windows Media Player. Help!!!! I only need to add a few more photos and the video for my Preschooler's Graduation will be complete.