to me. Are the files there but all the cells are blank, or the files themselves missing as suggested by the title and the last sentence of the post? If the former, then Kees is right -- your data are lost and you must go back to square one.
If, on the other hand, it is the files themselves that are missing, you have a fighting chance. If the files were accidentally deleted, they may still be the Recycle Bin and can easily be recovered.
If the files may have gotten moved to a directory you are not looking in, do a search over all drives for their names. If you don't include the extension with the name, this will also find them if the extension was changed as suggested by rcgyuk.
If the file name may have been changed, search on some bit of text within the file. Make the search text as unique as possible to reduce the number of false hits.
Hope this helps
Frank