Hey! I had a similar trouble after updating to iOS 11.2. Try the restart as Tartarus suggested, it can be a random glitch. If this won't help, check if there is any app or process running in the background, maybe there was a backup in progress or some heavy update.
In the worst scenario, it can be a corrupted iOS update (like in my case

). If you won't be able to fix it with steps above, ask Apple for help. Also, if you have a recent backup that was made before the update you can try restoring it (or just reset device to factory and restore this backup, should fix almost any iOS-related issue). Hope this helps!