You will see nothing in the library but if you go into the bryce folders everything will still be there, just import them back in where you had them. A cause for this happening, i'm only going on what i figured out. Is you have a large file in the library and you either try to export it or load it into bryce which causes bryce to "Not Respond" commonly called crash and the file library will empty like yours did. When i import objects into the Library i always put a B or S at the end of their names. B =big, S =small i do this so i can export the large files first.