maybe he just reversed a faces in MED ... why you are getting panic about a picture ... xD, ok here is the way u can solve it:

Open MED.
Switch to triangle mode.
Select a house, or whatever it is.
Copy and Paste it into SAME position.
And then click on the button 'Flip Normals' in some toolbar.

If that don't solve a problem, you really should post an image ...

EDIT: of course, when you open MED, you should load your model first, sorry, I really forgot to write it ... I think you will forgive me