Some things you can try. Do you have access to another WiFi network where you can try? That has worked for several people.
Try a restart.
Restart your iPhone, iPad, or iPod touch
Try a forced restart.
If necessary
Do a backup and try a factory reset. Then restore from your backup.
Restore - If you see an error when you update or restore your iPhone, iPad, or iPod
Backup - Restore your device from an iCloud or iTunes backup.