Apple is addressing the error 53 hubbub with tips to restore an iPhone or iPad via iTunes.
If you recall, error 53 became a big issue after Apple customers essentially bricked their phone after getting a screen replaced. The issue was that Touch ID's fingerprint data is stored in a secure enclave. Unless that data is revalidated Apple shut down the phone.
Welcome to the security feature vs. usability debate. The other issue was that some of the error 53 victims were folks who had screens replaced by third parties and not Apple stores.
- Get the latest iTunes.
- Force restart.
- Restore your phone via iTunes.
- Call support if that doesn't fly.
Apple will issue an update to iOS 9.2.1 so users can update iPhones through iTunes only. The update won't enable Touch ID again, but that's a security issue.