This usually happens when you moved file or when the files have been moved for you. Or when a folder name was chanded
If for exemple you first open a VI from a temporary location, then open you library, the path may be internally changed.
One fast way to rconnect them is to first open the VIs from the good location, then open the VIs that are calling the first ones, then save the modified VIs
It would be nice if you take the time to rate this answer
http://www.machinevision.ch