So it looks like you have a couple of issues here, let me address them individually. If you have a backup in either iTunes or iCloud, you will want to apply that backup to your iPhone in order to get your contacts back. The article informing you on restoring from your backup can be found here:
As far as the songs go, you will probably want to try deleting all of the songs off of your phone and resyncing them onto your iPhone.
The car stereo issue is a little more intricate, and has a whole article to itself outlining troubleshooting steps. You can find that article right here:
Hope that helps, and thanks for using the Apple Support Communities!