Where are your files located?
Are they on your C drive or on another device or drive?
When you first loaded your Project, did PrE ask you, "Where is file ______.avi, etc.?" If so, it usually means that the file(s) is not where it last was, or has been Renamed.
As Steve asks, where are these files currently located? Where were they located, when you Imported them into your Project?
Something has likely changed there.