Do other people use your computer? Any chance that someone else might've deleted your files, or updated your version of LW so that files are saved to a new location? Just in case, check out this thread: If you've updated LW and can't find your downloads!

Use your computer's search to look for some of the file names. If you can find the files, then you can add them back into LW's saved or shared folder & they'll show up again in your library.

And just in case they've been deleted, check out this thread for file recovery advice: DELETING FILES FROM LIMEWIRE LIBRARY-DON"T! - File recovery advice!