Even if I had a bad feeling about it I would start by assuming this was an honest mistake. Maybe something has happened suddenly in their private life, so thread carefully.

Try contacting your client through some other means; phone or physical mail marked urgent. Maybe someone else at your clients company if not a one man shop.

Make sure to get feedback on the work you've done so far if you get hold on somebody, to make sure it is not a disagreement about the work done.

Good luck!