I don't think you have to worry about a virus or anything like that being the cause of the problem, and it's too bad that you lost all that hard work.

Just a word of caution for the future, though: you should never try to open any of the files that have the .ldw extension. The only program that knows what to do with them is our game. Windows will not know what to do with them, and if you attempt to do more than just back them up or copy them, they may become corrupted and unusable. Borg's suggestion above is a very safe way to back them up, as is following the procedures here in the Guides Section.