CarPlay stopped working with Sony AX4000

Since you've disconnected and forgotten the device, most likely it needs to be set up again. Follow the instructions here: Connect iPhone to CarPlay - Apple Support

Since it doesn't support wired CarPlay:

If your car supports only wireless CarPlay: Press and hold the voice command button on your steering wheel. Make sure that your stereo is in wireless or Bluetooth mode. Then on your iPhone, go to Settings > Wi-Fi, and make sure that Wi-Fi is also turned on. Tap the CarPlay network, and check that Auto-Join is turned on. Then, go to Settings > General > CarPlay, and select your car. Check your car manual for more information.

Hard to say if the issue is your phone or a problem with the head unit. If you have anyone else with an iPhone that you can connect try to see if it works for them. If it doesn’t it’s more likely the head unit. You didn’t say if something happened to make you suspect the phone (for example, did you update iOS and then the issue started?). Check the user forums for your stereo to see if others are experiencing issues or have any ideas that may help….because if it is an issue related to the stereo they would be better equipped to offer relative suggestions for troubleshooting.

Sounds like you're trying to use wireless CarPlay. Does wired CarPlay work (i.e. USB connection between the phone and the stereo head unit)?