Going to throw out some random stuff, as I have not run into this issue.
Running latest iTunes on Windows 7 or better?
Tried different cable or sync via wifi? Sister recently had issues with connecting to iTunes, and after doing updates, seemed to be issue with some squirrelly Apple cables: my Amazon Basics, her hubby's after market, and one Apple cable in different room worked, but the everyday cables in the wall o' chargers had issues connecting to iTunes (all Apple cables that came with the gizmos).