Scheduled graduate admission interview never occured
I would send another follow up email 1-2 weeks after the first one you sent. Inquire politely, and offer to reschedule the interview. However, do it in such a way that they're allowed to save face. Meaning, phrase things in way that don't inadvertently point a finger at them. It might sound silly to placate egos, but...welcome to academia. :-/ 

If nothing comes from it, I would write this program off. As Astronat said, this is pretty poor behavior, and I would add that it might be indicative of what's to come if you were to be part of that department.