the misty stuff you saw coming out of the vents was freon in its liquid form. If your A/C is not working anymore, then you can guess the evaporator which sits inside the heater box is leaking. However changing the timing belt would not have caused this, so don't expect the shop that worked on it to tell you any different. When replacing the timing belt, it is not necessary to loosen or remove the A/C compressor, so consider this a "one in a million" coincidences.