While driving 15-20 mph down a long gravel driveway, I had to go around a slightly left curve. Then the road straightened out. The steering wheel locked and I couldn't move it to the left and could not get out of the curve. The car continued heading right, no matter how hard I tried to pull on it to go left. It just wouldn't budge. I continued to go right, off the driveway, and into some trees. At that point, the steering wheel was still locked. I turned the car off and back on and then it released so that I was able to back out from the trees. Before I take it to the repair shop, what caused this and what will fix it?
When a vehicle loses power assist in the steering it can definitely be a cause for concern. The 2015 Q5 is equipped with electro-mechanical power steering. This type of system uses electronics to provide the power assistance to the vehicle making the wheel easier to turn. When a failure is found you can lose the power steering assist, making the wheel very hard to move.
Audi issued a recall for some 2014-2015 Q5 vehicles for a power steering issue similar to the one you have described.
If the recall is unable to help you, have your vehicle inspected by Aa qualified technician, such as one from YourMechanic. They will be able to diagnose the issue with your steering wheel firsthand and perform any repairs required.