Wtf

Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:

• Press and quickly release Volume UP button
• Press and quickly release Volume DOWN button
• Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF). 

Axel F.

Sorry, but without a functional display, your can't start the iPhone.

You'll have to get it serviced first, by visiting an Authorized Apple Service Provider next to your place, or contacting Apple Support.

Apple - Support - iPhone Repair and Service

Apple - Find Locations