Visit an Apple Store or an Apple Authorized Service Provider next to your place and let them examine the phone. Maybe they can figure out what caused the damage.
But you might have to pay for a new screen.
Find an Apple Authorized Service Provider
iPhone - Contact Support - Apple Support
Repair and replacement costs depend on your iPhone model and your AppleCare product coverage. Apple will run a diagnostic test to determine whether your iPhone has experienced additional damage. If it has, or if it needs other repairs, you may need to pay an out-of-warranty service fee. Accidental damage isn't covered under the Apple One Year Limited Warranty.
Model |
Screen repair cost |
iPhone 6 |
$109 |
iPhone 6s, iPhone 6 Plus, iPhone 5s, iPhone 5c, iPhone 5 |
$129 |
iPhone 6s Plus |
$149 |
Plus a $6.95 shipping fee, if required. Fees are in USD and exclude local tax. Pricing is for service through Apple. The final service fee we charge will be determined during testing. Pricing and terms vary for service through an Apple Authorized Service Provider.
copied from Service Answer Center - iPhone