Check the hoses to the heater at the bulkhead, see if they are both hot, if they are I would suggest that the airmixer flap in the heater isn't working, if its mechanical maybe the cable has become disconnected, if its electronic, failure of the flap motor is quite common. If one or neither hose is hot you likely have an airlock as previous post, disconnect one hose and pour coolant into the header tank until it flows from both pipes.