My heat stopped flowing into the cabin I checked.

If they were frosted, that tells me the AC was running. If you had the defroster on, that could have happened. Check the following:

Is the engine running at a normal operating temp? If not, have you checked the thermostat?
Are both heater core hoses getting hot? If they are, check the blend air door in the vehicle.

With the defroster, the AC runs to dehumidify the air in the vehicle. That is why you saw frost. As far as the heater hoses, there will be two of them. They are aprox 3/4 inches in diameter. Both will run to the firewall where they attach to the heater core. You won't see the heater core because it is on the other side of the firewall.

With the engine running, warmed up, and the heater on high, feel both hoses. Both should be hot. If they are, that tells me the heater core is flowing and you should have heat. If you don't have heat and both are hot, there is a blend air door that determines if you get heat or AC in the vehicle. It must be stuck or the actuator isn't working.

Were they rubber hoses? If so, is there a chance that the coolant is weak and it froze?