The first thing to check is the coolant level in the radiator. If the radiator is low it will cause the vehicle to no longer put out heat. Top it of and repair any leaks. If the Thermostat is stuck open there will be very little heat coming from the dash and the Temp Gauge on the dash will read low. If the upper and lower radiator hoses are the same temperature then the thermostat is opening. If you have one heater hose hot and one that is cold then either the heater control valve isn't working or the heater core is clogged.