“Apple has just released [then quickly pulled] iOS 8.0.1, a bug fix update for iOS 8,” Rene Ritchie reports for iMore.
“Unfortunately, it’s left some iPhone 6 and iPhone 6 Plus with Touch ID and cellular connection issues,” Ritchie reports. “Reboots don’t seem to fix it. It’s possible Apple can fix and re-issue the update quickly, or a workaround will be found shortly, but if you’ve already updated, and you’re affected, here’s what you need to do.”
Easy-to-follow instructions here.