A heater that suddenly blows cold usually means low coolant, a stuck thermostat, or trapped air in the system.
Engine temperature gauge reads normal
Started suddenly, not gradually
Not enough coolant reaching the heater core means no heat, even if the engine runs fine.
The engine never reaches full operating temperature, so the heater stays lukewarm at best.
An air pocket can block coolant flow to the heater core after any coolant service.