Why would the heater in my land rover discovery 300 tdi suddenly start blowing cold and there is water in it?

if there is a lot of pressure in the header tank,you got headgasket trouble or a cracked head. when cold start car up take water pressure cap off look for bubbles and a smell diesel.|||If you mean there is water in the cooling system (as normal) then the heating system should work fine. My only suggestion would be that the linkage between the heater dial on the dashboard and the heater unit has come apart, this is usually just a cable like a bike brake cable. If you mean the water is coming though the ventillation system then it would appear you have a leak, which is why there is no warm air coming throught the car. The heaters use the hot water in the cooling system to warm the car, so if there is no water in the cooling system there is no heat,|||Try your thermostat. it may not be opening or possibly wour water pump is defective|||Check your heater core. I think it is bad. My truck did the same thing but anti freeze instead of water came out.