This info. would have helped in the diagnosis. Turn on the vechicle and run till it warms up. Then touch the inlet and outlet pipes on the heater core. They should both be warm or hot. If the outlet is a lot cooler than the inlet, your heater core is plugged or the valve has failed. These pipes will get heated up if the coolant is flowing. They won't heat if the coolant is not circulating. Best to take to shop for the flow test.