GoKawiilThe weak link between your smartphone and your car is the connection, wireless or wired. For wired users, you can start troubleshooting by checking that your cable is connected properly and free from any damage. If there's no visible problems, the cable or port could still be fried — switch it to another USB port or switch the cable out for a replacement. You don't want to be using a cheap USB cable that's designed for charging for this job, as you'll need one that supports data transfers over USB. This Google guide recommends that you use a cable shorter than six feet and to ditch any cable extensions, too.
If you're using Android Auto wirelessly, you'll need to make sure that both Bluetooth and Wi-Fi are switched on. If they are, and it looks like you're connected, then disconnect — you want to force it to make a manual reconnection to your car.
That'll also mean making sure your car is in the right app or menu for Android Auto to connect and become visible, which might mean selecting it on your car's infotainment screen. If you think the car is the problem, you can always rule this out by using another Android phone if you have one spare.