1988 Honda CRX No Heat

Heater problem
1988 Honda CRX 4 cyl Front Wheel Drive 204000 miles

THis is an HF model in outstanding shape. Preventive maintenance done (I've had it since new.)

During a trip to CA and back to CO last week, the heater suddenly began to blow cool air only, i.E, no gradual reduction in heat noticed indicating a heater core blockage. Changed the thermostat and visually inspected heater hoses to include the in-line temp selector valve for blockages--non found. Still no heat. Thoughts?