Now as for this mysterious pendant with the flash drive in it, the attic scene at the end totally blew it for me. That brings it to a level where we should probably expect aliens to land during the next couple episodes. It wasn't even like she plugged the pendant into the computer and it had some stored power, it's got 'magical proximity power'.
I am under the assumption that some group invented a device that stops electrons from flowing in wire, perhaps a strange energy field that dampens the flow or something. The field expanded out from a device to cover the world. This would explain the electricity going out as you watched it spread instead of instantly going out. The small devices shown with the USB drive block the field for a limited area, allowing electrons to flow.
Of course even this theory has a lot of issues, like how are people still alive if electrical impulses from the brain are not able to get down to the heart? It is simply a fantasy scenario that one has to accept for the show.