If at all possible see if the screen asking for your pass code comes up and enter your pass code to get into the phone and turn off fingerprint ID for the time being.
These are some of the reasons this can happen:
If you've just restarted your device
If your fingerprint isn't recognized five times in a row
If you haven't unlocked your device in more than 48 hours
If you've just enrolled or deleted fingerprints
If you're trying to open Touch ID & Passcode in the Settings menu
Here are some things you can try:
Make sure that you have the latest version of iOS.
Make sure that your fingers and the Home button are clean and dry. For the Home button, use a clean, lint-free cloth to wipe off any dirt or debris.
Your finger should cover the Home button completely, touching the surrounding metal ring. While Touch ID is scanning, you shouldn't tap too quickly or move your finger around.
If you're using a case or screen protector, make sure it doesn't cover the Home button or the surrounding ring.
Go to Settings > Touch ID & Passcode and make sure that iPhone Unlock or iTunes & App Store is on, and that you've enrolled one or more fingerprints.
Try enrolling a different finger.
If you can't enroll any of your fingers, take your device to an Apple Retail Store or Apple Authorized Service Provider, or contact Apple Support.
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