Scott's USB lightning cable has stopped working in his car. Says there's no music available on the car screen. Leo says to try a new cable to eliminate the possibility that the cable is going bad. Also, there may have been an update to CarPlay that isn't supported by the car stereo head unit. Apple may have changed something in a recent iOS update. Another option is to turn the phone off completely and then turn it back on. That'll reset the network settings. Check the stereo's website under "support" to see if there's any technote on the issue.
Lastly, there could be a connection issue on the lightning port. That port collects lint that can prevent a full connection of the lightning cable. Blow it out with some canned air. Also, use the lightning to headphone adapter and see if music is audible. If possible, then it eliminates the phone as the issue. He can also try a different iPhone connecting. Or try another music source other than Spotify. Try and eliminate every link in the chain.