My thumb print won’t read. What do I do

Your iPhone's passcode is still available as a backup method, so tap on "use passcode" if it asks then tap in your passcode. If your Touch ID fails too many times you will also get asked to enter your passcode.

When you get into the phone by entering the passcode you can go into Settings and reset your Touch ID by re-enrolling a finger. It is not unusual for TouchID to fail to read a fingerprint (dirty, wet, abraded fingers, maybe even a minor injury will all interfere). I suggest enrolling several fingers so you have a backup. (Note that to get into and change TouchID settings you MUST know your passcode).

Try these steps

Make sure your fingers and the Touch ID sensor are clean and dry. * Use a clean, lint-free cloth to wipe off any dirt or debris on the Touch ID sensor. Your finger should cover the Touch ID sensor completely. While Touch ID is scanning, you shouldn't tap too quickly or move your finger around.

If Touch ID isn't working on your Mac - Apple Support (IN)