Hmm...perhaps there is a map file that's either missing or corrupt. Check the forum you downloaded the map from to see if anyone else has had any problems with the map, and if you find nothing, delete the map files and re-download it from the internet.
My other thought is that possibly your computer can't handle loading this particular map, whether it's simply too large, there are too many objects on the map, etc. Other than that, I don't know.