My 2005 Monte Carlo's heater blows cold air after AC compressor was replaced

What is your question?

I had the heater in my 2005 Monte Carlo repaired last winter and it worked fine. 3 months ago, when it was the summertime and hot, I needed to have the compressor replaced for my AC to then work. The AC worked perfectly for the rest of the summer, but it's cold again now and the heater doesn't work! In fact, when I turn on the heater, it blows cold air! I don't know WHO to blame for this, the first mechanic who repaired my heater last winter or the AC repair guy from a few months ago. Nor do I know what's causing this problem now. If I had to guess, I'd say that whatever happened during the replacement of my compressor caused my heater to stop working but I need to have a better idea of what this problem is so I can go back to one of these two repairs guys to complain about it and have this problem fixed.

What makes this problem better or worse?

When it's freezing outside and the heater blows AC air

How long has your 2005 Chevrolet Monte Carlo had this problem?

Since getting my AC repaired 3 months ago