Hi!
I see that this is a widespread problem. You might want to try a forced reboot first, but if it doesn’t work the way you described, move on to the next solution. Anyway, to make a forced restart:
Press and release the Volume Up button quickly
Quickly press and release the Volume Down button
Long press the Side Button (power) until the Apple logo appears
The second solution is to restore, configure and update your iPhone again, all via Finder or iTunes. To restore:
If you have a Mac with MacOS Catalina 10.15 or later: Connect your iPhone to your computer and open the Finder. At this point you need to put the iPhone in recovery mode. To do so:
Quickly press and release the Volume Up button
Quickly press and release the Volume down button
Long press the side button (power) until the Apple logo appears
At this point select the device in the list on the side, press on "General" and then on "Restore".
If you have a Mac running MacOS Mojave 10.14 or earlier or a Windows PC: Connect your iPhone to your computer and open iTunes. At this point you need to put the iPhone in recovery mode. To do so:
Quickly press and release the Volume Up button
Quickly press and release the Volume down button
Long press the side button (power) until the Apple logo appears
At this point select the device in the list on the side, press on "Summary" and then on "Restore".
Now you need to reconfigure your device. At the end update again, but for safety via Finder or iTunes. To do so:
If you have a Mac with MacOS Catalina 10.15 or later: Connect your iPhone to your computer and open the Finder. Select the device in the list on the side, press on "General" and then on "Check for updates". At this point press on "Download and update".
If you have a Mac running MacOS Mojave 10.14 or earlier or a Windows PC: Connect your iPhone to your computer and open iTunes. Select the device in the list on the side, press "Summary" and then "Check for updates". At this point press on "Download and update".
Your device should work now.
If the problem still persists, contact Apple Support.
I hope I helped you!