Several Windows Phone devices, such as the Nokia Lumia series, may have trouble syncing with Zune. The device is correctly seen in Device Manager but Zune simply says it can't connect to the device, and that you need to turn it off and on again. Removing and reinstalling Zune does not help, nor does rebooting the phone.
Most of the time, the problem is caused by an invalid certificate. This usually occurs after updating Zune, or after installing other Windows Updates. You can recreate the certificate by stopping Zune, removing the certificate from the user's certificate store, then start Zune again.