I don’t have any experience with your make/model, but some of those air vent controls in newer cars are controlled by vacuum motors. If so, either the vacuum motor has stuck or failed, or it isn’t getting the correct vacuum like it should. Ask your mechanic to look under the hood to see if there are any vacuum hoses – especially one that goes through the firewall – that have become disconnected, broken, etc. He might be able to test the vacuum motor too by attempting to pull a vacuum on the hose that goes to it. Could tell if it is leaking or not. Is the car idling ok? If it has started to idle rough, that could be a symptom of a vacuum leak in the vacuum motor.
The other idea to consider is that a stick or leave has become stuck somewhere in there. Ask your mechanic, maybe there is a way to blow air through those vents using an air hose to free it up.
It’s a good idea as part of preventative maineanance to keep the airflow inlet screens (usually just under the windshield) clean of debris, especially if you park under trees regularly. A shop vac takes care of this in a jiffy. Best of luck.