Repair the leaks and the heat will return.
A. Obtain a loan free radiator pressure tester from the local auto parts stores such as Autozone, O'Reilly or PepBoys. Follow their instructions and pump it an adequate air pressure 12 -15 psi or the allowances on the radiator cap. Observe the leaks and perform the repair.
Note: do not OVER-PRESSURE than the pressure allowed in the radiator cap.
B. Insufficient coolant , trapped air in the cooling system might not have enough coolant to fully fill the heater core.
The hot coolant flows from the engine to the heater core through the "inlet" hose. After circulating through the core, the coolant returns to the water pump through the "outlet" or "return" hose. Both the inlet and outlet return hoses should feel hot when the engine is at normal operating temperature and the heater is on.
1. If the both heater core hoses were cooled, then this vehicle might have an insufficient coolant in the cooling system.
2. If the heater core inlet hose is hot and the outlet hose is cool, the heater core might be clogged.
Note: check the radiator coolant level and top it off as needed.