Black unresponsive screen

If your screen is black or frozen

If your screen is black or frozen, you might need to force restart your device. A force restart won't erase the content on your device. You can force restart your device even if the screen is black or the buttons aren't responding. Follow these steps:

• iPad models with Face ID: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Press and hold the Top button until the device restarts.
• iPhone 8 or later: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Press and hold the Side button until you see the Apple logo.
• iPhone 7, iPhone 7 Plus, and iPod touch (7th generation): Press and hold both the Side (or Top) button and the Volume Down button for at least 10 seconds, until you see the Apple logo. 
• iPad with Home button, iPhone 6s or earlier, and iPod touch (6th generation) or earlier: Press and hold both the Side (or Top) button and the Home button for at least 10 seconds, until you see the Apple logo.

If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple ...

https://support.apple.com/en-us/HT201412

If a forced restart didn't solve your issue, since your device is still on warranty, I would have it inspected at an Apple Store or Authorized Service Provider:--> Apple – Location Selector

Regards

Giulio

Warranty is determined by serial number. No need to bring any proof. Also, they will examine any device in or out of warranty, and will explain your options from there.

Well, if it is still covered by warranty, they should help you.

You can check your device's coverage here:--> Check Your Service and Support Coverage - Apple Support

Regards

Giulio