Editing Problem: Video Files Cannot Be Edited, Saved Project

I have saved a project to return working on it later on. I opened the Saved Project. It showed all the videos present (trimmed and untrimmed etc.) and it did not display any typical, problematic "missing file." In other words, everything looked normal. However, half of my videos can be edited normally while other half cannot. I clicked on one of the videos, and all the options on "Video Edit" is off (the grey, off color), and I was not able to preview the movie. When I attempt to "Save Movie" it suggest I check "File Preparation" which was already checked.