heater stopped working

I have and 06 kia sorento. recently the heater stopped working, I changed the thermostat and refilled the coolant. The heater worked great after that but the a/c quit working. Now the heater has stopped working again and the coolant levels are fine, what could it be?

We have had no problems with this vehicle until recently now it seems to be one thing after another. I have enough mechanical knowledge to do the work but not enough to diagnose these newer vehicles,

Any help you could give would be greatly appreciated.