While the WE very rarely does do buggy little things like corrupting a map, if it's not just a rare buggy situation, we're certainly not going to be able to isolate what's wrong with as little information as you've given.
What were you working on in the WE? Were you doing anything else while saving? Did you mess with the map archive file? Have you used the Test Map feature, played the map with anyone else, or made any backups?