Chances are your imports were corrupted. The most common cause for this is testing your map without saving it first. It will save to Warcraft III/Maps/Test/WorldEditTestMap instead of to the directory you had your map file in.
You can check to see if WorldEditTestMap has working models if you haven't tested a map since then. Otherwise, you'll have to replace all of the imports with non-corrupt versions by right-clicking and choosing to replace the files in the import editor.