Nexus 6 Not Showing up in "My Computer" or ADB with Debug Mode Enabled
Installing Universal naked drivers shall solve your problem. Assuming you are skillful enough installing drivers manually as you intend to make use of ADB, here's a reference link to Universal Naked Drivers: http://forum.xda-developers.com/google-nexus-5/development/adb-fb-apx-driver-universal-naked-t2513339

It generally supports all Nexus devices(probably all), if it doesn't help, uninstall the current installed driver from Device Manager and replug your Nexus device to make it install its drivers again.