When I was stationed in San Diego I had a 72 Swinger, one rare day it got cold enough to need a heater. No heat. usually that means you are low on coolant, but not in my case (did you check yours?). Turns out the core was plugged.

All I did was disconnect the heater hoses and use a garden hose to flush one way till the water ran clear, then the other way till it ran clear. Took a couple of times going back and forth till it ran clear both ways from the get go. Plenty of heat after that.