Q: heat stopped working, only blowing cold air. checked and replaced fuses but no change

Two common causes of no heat, as you describe the circumstances, are a fault with the heater control valve and/or a problem with the blend door in the under dash ventilation system being stuck in position or inoperable. There are other, more unusual causes such as the heater core being plugged, trapped air, and so forth but we need not analyze them here as if you request a no heat inspection, all of the possible causes will be looked at. Heater control valves are usually fairly straightforward to replace, although the cooling system has to be partially drained. If the fault is in the blend door, that will have to be physically accessed and it is usually buried in the dash.