Really tough to hear. Here's some inspiration: http://techcrunch.com/2013/10/19/phil-libin-evernote/ I remember hearing about how they were saved last minute by a customer/investor at startup school 2 years ago.

Any way to woo the customer back? Show up at their office tomorrow morning and convince them to stay or at least keep some of the account with you? Maybe convince them to give you a bit of a window? They might have no idea that leaving your company will cause your company to fold.