If your Android auto not working you might need to install all system updates, as well as the latest updates for all Android Auto compatible media and messaging apps, before you can keep using Android Auto. Check Google Play for updates and learn how to update your apps. If all your apps are updated, try turning your phone off and back on.
In most of the cases it’s missing app or Android updates that causes Android auto not working, updating your system and apps should be first step following by checking if you have Android auto installed.
In some cases Android auto also started working after phone or application itself was restarted, but generally that’s not the reason for Android auto not working.