Ios 14 update

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). 

(2) If the above does not work then - Follow the instructions in this article by Apple ➔ If your iPhone is Non-Responsive or Frozen and see if that helps.

(3) If that does not help your phone might need service ➔  Contact Apple for support and service - Apple Support

Axel F.

It may also be an incompatibility issue between PUBG and iOS 14 (that older versions of the app don't support the newest iOS system), try updating the app if you didn't do so yet.