Recover Windows 8.1 and get the built-in Windows Product Key

I have a Dell Venue Pro 8 tablet. I tried to factory restore last week, during the restore, the tablet hanged and can't restart.

I built the recovery USB drive before the restore, but the system ask me to plug AC power to restore via USB drive. However, this device has one USB port only, I can't plug the restore USB drive and AC power at the same time, even I tried a USB hub.

What can I do to restore the Windows 8.1? And how can I get the Product Key in a built-in device?