Footage became unlinked and now gives me error when I try to link it.

[andrewm54607989]

I had a cut of my film and was making further edits and renders and then all of a sudden on a render it stopped and said I needed to save to a different location.  After I did, a majority of my footage dissapeared.  I went to my back up and restored the footage and it still wont work with some clips.  What is going on?

[basil1891]

Are you working with external/removable/network storage? 

How much free space are there on a cache and export disks?