my car does that occasionally too. most of the time the best fix is to simply restart your iPhone.
i have however found that if i do not have time or just do not want to restart the phone my quick workaround do the following...
- start a song playing in the Music app then plug into the USB port.
- if that didn't work, then sometimes i have to leave it plugged in and start playing a song again, then switch back to the AUX input on the car.
usually this gets me up and running quickly! kinda like push starting an old manual transmission car 😎