Hi,
I'm not sure what your specific issue is, but I think we can do some things to try and isolate the problem. First try restarting both devices, I know you said you already did, but I just want to make sure you have restarted both devices as this tends to fix a lot of issues.
Next, are you trying to sync wirelessly or with a wired connection? If wirelessly, attempt to connect the devices with a lightning cable and see if that fixes the issue. If you are already using a wired connection, try a different cable. It is possible that the cable is somehow internally damaged and is causing connection issues.
If none of that solves your issue, next you should try to clean the lightning port on your iPhone, often the port can gather pocket lint and dust that might be preventing the lightning connection from seating properly or the terminals from properly connecting.
Follow this Apple Guide for cleaning your device.
You may want to follow this guide to clean your iPhone's lightning port, however do so at your own risk and very carefully as you may accidentally damage the sensitive pins inside the port and permanently damage your iPhone.
Hopefully that helps!
