restore from backup

Here are two relevant support pages that may be helpful with troubleshooting the issue:

If you can't back up your iOS or iPadOS device to a computer or restore it from a backup – Apple Support

If you can't update or restore your iPhone, iPad or iPod touch – Apple Support

Try these steps https://support.apple.com/en-us/HT204184

make sure you are following the steps right if these steps won’t work contact Apple support