I have libra 4.2 and this happened shortly after the upgrade. I have several hours of work here. There was something that popped up I remember asking open with and not sure what I pushed but it never asked before. Every morning for the last year up till two weeks ago I made an entry in one of the spread sheets and then corruption.
First close all your open applications. Then double-click the icon of one of your spreadsheets. What application loads? Do you get any messages? If the file loads, is it corrupted?