Perhaps you have installed the latest iOS version: iOS 11 beta on your device. But as a beta there are several issues. Some users say that their iPhone does not charge after updating to iOS 11, call is not possible etc. It is possible to roll back from iOS 11 beta to iOS 10.3.2 or 10.3.3.
Step 1. Go to the site
On your computer browser, select the appropriate IPSW file according to product, type, system version. Click on the button Download (Download).
Step 2. On your device, go to Settings> iCloud and turn off Find My iPhone / iPad / iPod touch
Step 3. Wait for the download to complete (this takes a long time)
After downloading the IPSW file, connect your device to the computer. Start iTunes and click on the device. Then click on the button Restore iPhone / iPad / iPod and select the IPSW file to start the downgrade.
You can also read the following articles:
How do I know if my iPhone is restricted under iOS 11.3 or New in iOS 14 updates.