Have you tried using "3uTools?" I had the same problem. 1.Download 3uTools 2.Install it(Obviously) 3.After Installation connect your IPhone in Recovery mode 4. In 3uTools select the option called "Flash & JB" 5.Select the option called "Easy Flash" 6. And after that you will see a list of available IOS versions for your IPhone. 7. Untick the option called "Retain User’s Data While Flashing" Beside Flash 8.Just tick the Circle of the version of IOS you prefer and click "Flash" If not, try visiting your local apple store
Do a hard reset. First turn off your device. And after turning off your device keep pressing Volume Up+Home Button+Power Button. Release the power button when device is on but just keep pressing Volume Up and Home button until you are in recovery mode. When you are in recovery mode already select Wipe Data Factory Reset by using the volume rocker to scroll down and power button to select now select factory reset and you should see a bunch of no's and only one yes scroll down to do yes and select it using the power button.
Hmmmm.... weird are you on Legacy Or UEFI? If you're on UEFI try switching to Legacy you might see your USB Drive Again And what program did you use to burn your Windows 10 ISO? and is the partition on the USB Drive active?