I'm having this same issue as well. My Electron was running fine in the field along with two other "identical" Electrons and this one seems to have fallen off the face of the earth. It is no longer publishing events. The others are working fine. I re-flashed the application for giggles and did a "particle update" just to be sure.
When I power on the device, it appears to attach to the cellular network and breaths cyan appropriately. However, I see absolutely no evidence of the device coming online in the logs. Nor do I see any evidence of the webhook being called via Particle.publish().
Again, this device was running for weeks just fine all alone with no one touching it. It went offline at 1:11AM EST today. Was there any special platform maintenance being performed at that time?
[update] This same thing may have happened to one of the other Electrons last week. I assumed it had gone offline for some strange reason, but it was breathing cyan just fine when I opened its enclosure. I disconnected the battery and power adapter and restarted it and it came back online just fine. However, it had been offline for a couple days.