This happened to me recently. I needed to download my iPhone photos and videos like I normally do periodically, but I was not seeing the device in Windows Explorer like I used to.
The solution was simple. I just need to:
1. Unplug the phone from the USB charger
2. Enter my passcode
3. Plug it back in
Why? When the phone is locked, Windows cannot detect the device properly. I hope this helps someone.