Apple Support may have run you through most of these steps, but just to be sure:
If your computer doesn't recognize your iPhone or iPad - Apple Support
Note that if you have installed the newer Apple apps - Apple Music, Apple Devices and Apple TV - then you will use Apple Devices to sync, not iTunes.
If that article doesn't help, post back with more details and someone may be able to offer further suggestions, though since Windows has so many components, many of them coming from developers other than Microsoft, troubleshooting connection issues remotely can be difficult if not impossible.
Regards.