Apple released iOS 8.0.1 yesterday and as many users rushed to update in hope that this update will fix many bugs present in iOS 8, they were surprised to find that the update, in fact, broke Touch ID and cellular reception, resulting in a “no service” status that could not be resolved with an airplane mode on/off routine.
This problem was reported by users owning iPhones 6 and 6 Plus, while 5S users reported that their update process when just fine. So it seems this update only affected iPhone 6 and 6 Plus users, nevertheless, you’re probably better off waiting for Apple to investigate on what went wrong and fix it in the upcoming iOS 8.0.2 due in the next few days according to the statement Apple released (check it out here) shortly after pulling 8.0.1 offline.
Apple’s fix, for the time being, is simply to downgrade back to iOS 8.0.