Apple CarPlay stopped working in one car the day after an iOS update. In another older car, it still worked. In the newer car, kept getting "accessory not supported" msg & the CarPlay icon on home screen was "greyed out" (disabled) despite USB cord & phone not having changed since day before when CarPlay worked.
Here is what worked for me to get Apple CarPlay working again although it require your car having a separate bluetooth music player function in order to work:
(1) The phone was not connected to the car with USB cord during this step:
Disconnected & then reconnected bluetooth connection from car to phone by going on the iPhone to BlueTooth, the "i" circle next to my car, and "Forget this Device" and then reconnecting it. Disconnected CarPlay as well by by going on the iPhone to General, CarPlay, MyCars and then clicking my car and "Forget this Car", but was not able to reconnect to CarPlay until Step 4 below because CarPlay icon was "greyed out."
This Step 1 may not be needed, but since I did do it, am including. Would suggest to skip this step & try 2, 3, 4 below and only if it doesn't work then come back and do Step 1.
I did also after Step 1 go into the older car and that is when I discovered that both Carplay & Bluetooth Music Player were still working fine in the older car. When I moved from the older working car back into the non-working car, the Bluetooth Music Player switched over automatically and started playing the music in the non-working car and that's how I accidentally discovered this fix:
(2) Used the bluetooth music player on my car home screen to play music using Spotify app on my phone. No USB cord connection still at this time. The music came in over the car speakers wirelessly.
(3) Once music was actually playing, only then plugged in the USB cord from phone to car.
(4) Now suddenly the phone was recognized and I could activate CarPlay normally, this is the point at which I re-established CarPlay that I had disconnected in Step 1. The music that had been playing via bluetooth wirelessly now switched to playing over CarPlay instead of wireless and other CarPlay functions like GoogleMaps were now working as well.
Another family member had a phone that also was suddenly not working with CarPlay in the newer car and this fix worked for them as well. They did also did do Step 1, but they never went into the older car.