If what you say is accurate, the only other thing I would try is reaching out to one or more of their investors / board members. It might not help, but as a paying customer, I'm sure they'd want to know (and can make a call and get your grievance bumped to the top of the line).

Since your representing that your business is in jeopardy, do what you need to do. It's great that you are sensitive to the plight of startups and don't want to do anything that might hurt their chances of success. However, they also have to live up to their end of the deal. If they aren't communicating with you, that's a huge problem.