I have a 2018 Silverado as well with 1200 miles on it and mine did the same thing this morning. Drove across town, stopped for breakfast, started my truck and had the steering assist issue pop up, will not move in reverse and no a/c. Engine was running but tach showed 0 rpm, also showed traction control was off. Steering wheel would not turn either. 45 minutes later, I was able to reverse out of the parking space and go straight to the dealer with it acting like nothing happened. They have it now and said that they were able to duplicate. They also said that it showed that communications dropped off a few times. They replaced the ecm and said that they wanted to keep it overnight so they could try it again tomorrow. We'll see what happens!