The Electromagnetic Pulse (EMP) Theory
This theory is one of the most popular among fans, positing that the “event” was some massive electromagnetic pulse that knocks out all the electricity in the world. It’s easily the most plausible theory, but some of the particulars don’t add up. For one, how the heck can the pendants turn things back on if they’ve already been fried by an EMP? Second, unless it's some type of sustained EMP, it should only kill old tech, not prevent new technology from being created. But the writers could always work around those finer points.
Odds: Somewhat likely.