Do you have your sidebar up?
Press CTRL & S together
You can also:
- try another cable or usb port
- create another user on your computer and test the issue in the new user account
- try on another computer
With issues like these you want to isolate it to being a fault with eiither the cable, usb port, user account, iTunes or PC.
In your case it doesn't seem to be a fault with the iPhone since the issue occurs with all your devices.
So I tried a different account and swapping out the cables. Even broke out as brand new one. No joy on Windows 8.1. I'm going to guess there was something in my Win 8.1 settings that prevented iTunes from syncing, since the computer recognized my iPhone, and even suggested I transfer the camera roll photos, but did NOT load iTunes.
SOLUTION: I finally got it to sync to the RIGHT (but not the LEFT!) USB port on my MacBook Pro with Retina running Mavericks. Why not the left port? Is there anything different about it? Works fine for other stuff.
Back in July, everything synced to my Windows desktop just fine -- but of course that was before iOS7 and Windows 8.1.
So I guess I should mark this solved. If anyone with a similar problem is reading this, I would say your best chances of syncing an iDevice running iOS7 to a computer would be to find a Mac running Mavericks and try ALL its available USB ports before giving up. Failing that, find a PC NOT running Win 8.1.