My phone is powered on but it is stuck on an app and will not close the app, I can not shut it down either.

Try a "forced" restart of your iPhone, using these instructions:--> Force restart iPhone – Apple Support

Regards

Giulio

Hello ~ Give this a try…it is an excellent prior post by *Axel Foley*

“(1) 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.”

~Katana-San~

See this Apple article: Close an app on your iPhone or iPod touch

Close an app on your iPhone or iPod touch - Apple Support