This problem happened twice for me and for my wife. I was backing up then suddenly the steering wheel locked and the wheels were unable to turning. Was there a delay time to get this thing works because after a minute and so both steering and tires work and back to normal. earlier today the same problem my wife encountered but, this time it took her an hour to get this thing work and back to normal.
No steering system warning light on? Sometimes something as simple as low charging system voltage can disable the power assist. This is probably something that you are not going to repair yourself.