Apple today posted the following support article:

Loss of cellular service or ability to use Touch ID after updating to iOS 8.0.1 on iPhone 6 or iPhone 6 Plus.

We have a workaround for you if you have an iPhone 6 or iPhone 6 Plus and you lost cellular service and Touch ID functionality today after updating to iOS 8.0.1. You can reinstall iOS 8 through iTunes by following the instructions below. We are also preparing iOS 8.0.2 with a fix for the issue, and will release it as soon as it’s ready in the next few days.

Follow these steps to reinstall iOS 8.0.

1. Make sure that you’re using the latest version of iTunes.
2. Connect your iPhone to iTunes.
3. Back up your iPhone in iTunes on your Mac or PC. iCloud backups won’t restore to earlier versions, including iOS 8.0.
4. Download the file below that corresponds to your device:
– iPhone 6
– iPhone 6 Plus
5. Select the file you just downloaded by doing one of these in iTunes:
– Mac: Press the Option key and click Check for Update.
– Windows: Press the Shift key and click Check for Update.
6. Press Update to install iOS 8 on your iPhone.

The Health app won’t work in iOS 8 after these steps. It will be fixed in our upcoming iOS 8.0.2 software update.

Related articles:
Rush Limbaugh: How did Apple miss the iOS 8.0.1 bugs? – September 24, 2014
How to get rid of iOS 8.0.1 and revert to iOS 8.0 – September 24, 2014
iOS 8.0.1 fiasco: Apple exec responsible for catching bugs before release has a ferocious door-slamming temper – September 24, 2014
Apple pulls iOS 8.0.1 after users report major problems – September 24, 2014
ALERT: Do not install Apple’s iOS 8.0.1; breaks cellular connectivity, Touch ID support for ‘large number of users’ – September 24, 2014
Apple releases iOS 8.0.1 – September 24, 2014