Yes, Huurka is right. What did you do to your map before it was working?
I'm sure you don't use NewGen Pack so first... DOWNLOAD NEWGENPACK. It's basically a much more powerful World Editor, which even backups your map 20-30-something times.
And whatever you do on the computer.. don't forget to keep backups. It costs nothing, it takes a few seconds and it can save your sanity.
I would also like to note that there is a backup map made by the original World Editor. It's called TestMap or something and it's in the "Maps/Test/" folder.
Next time please use the search function or read the FAQ because it's annoying to /facepalm over the same problem every day.