When I connected my new iPhone 7 to iTunes on my windows pc it does not appear in the left sidebar so I am unable to drag any music files across from my library to the phone.
Unplug your phone from your PC.
Shut down the PC.
Plug your phone in to your PC, and turn the PC on to see if Windows will download any new drivers needed for your phone.
If that does not work, uninstall iTunes and then reinstall. There's very specific steps needed for uninstalling iTunes from Windows, so make absolute certain you follow the directions exactly to the letter also ensuring you read the entire page first before you begin the steps;