Earlier today, Apple released an update to iOS 8 that promised to fix many of the issues that were in the operating system after its general release on September 17th. Unfortunately iOS 8.0.1 was not properly bug tested and caused owners of the brand new iPhone 6 and iPhone 6 plus to be unable to use TouchID or connect to a cellular network. Fortunately, the issues were discovered before must people were aware of the update and Apple has since removed the update.
If you were unlucky enough to install the update and you have one of the new iPhones, iMore.com has instructions for restoring your phone to iOS 8. If you have not installed iOS 8.0.1, do not install it until any issues have been resolved by Apple.