My iPhone 5s will not sync with my iTunes anymore!
I have only seen this with beta versions of the OS and/or iOS. 

The first step is going to be to reproduce the problem. 


Connect everything power off the iPhone 
Save your work on the Mac and shut it down as well 
Power on the iPhone and then put it in airplane mode 
Start the Mac and fire up iTunes (only iTunes if possible )


Then keep good notes on what you see - all dialogs and exact text, what operation causes the issue and the times. From there, you can look over access logs and iTunes logs on the Mac and/or get support involved to read the legs from the iOS device if needed. 

Feel free to comment or ask a follow on question once you have it pinned down. For me, the restarts fixed the issue or it was a server side issue and not reproducible past the time Apple's servers were busy or slow.