It throws you for a loop when clients just up and leave doesn't it? Sorry this happened to you, and hope you replace them soon.
That said, we have to learn from these incidents so I hope (if you didn't) next time you get a deposit. I would also get them to initial all payment requirement portions of your contract so they will think twice before they pull this type of stunt. Obviously, they have been planning on leaving and have someone new and did not have the courtesy to give you proper notice. Do they not get they are screwing up the income you depend on?
Good riddance!