Heater blows cold

Check the coolant top and bottom hoses and the radiator.
It might be blocked or the pump might be gone.
If the rad has an even temp and the pump appears to be working there's a bleeder valve in the top hose,
crack it open when it's hot in case it's vapour locked and let the gas/steam out.
The 2 smaller diameter hoses that run into the bulkhead are the heater feed and return,
both should be hot.

A thermostat jammed open wont help either .If there is a heater control valve fitted , check that also .