Lightroom: Just starting to love LR3, but suddenly perplexed & frustrated.

Suddenly all the photos in my library come up with a black band message saying " The file named ............... Is offline or missing". The photos; NEFs, or Tiffs are there on the drive and can be accessed via Nikon ViewNX2. Any ideas what this is all about? I removed the program and re-loaded, to no avail. Thanks for any help.
Rick