Try a forced restart:
press vol up and then vol down and then press and hold the side button till the phone shuts off. Keep holding and release when you see the Apple logo.
if it doesn’t work, you can double check if that backup to iCloud was successful, well you could perform a factory reset:
press vol up and then vol down and then press and hold the side button till the phone shuts off. Keep holding even when you see the Apple logo, and release when you see the iTunes/computer logo. If the Apple logo flashes and disappears, reattempt this again while connected to iTunes.
connect your iPhone to your computer and launch iTunes, and on iTunes click Restore iPhone. Later you can restore your iPhone from a backup.