My truck was blowing heat yesterday and now on cold day blowing cold air.

What is your question?

This is the first time this has happened. We recently replaced the thermostat and thermostat housing. It was very cold this morning but even after the temperature guage was reading that the engine was warmed up it still blows cold air. I put a mixture of 50/50 antifreeze in the radiator. What could possible be wrong?