First try this:
Hold down the Home and Wake/Sleep buttons at the same time for about 15-20 seconds until the Apple logo appears. Ignore the "Slide to power off" text if it comes up. You will not lose anything.
If that does not work then do a full restore:
Please follow the instructions in the article below. The process is rather straight-forward as iTunes will guide you along the way. You will have to disable Find my iPhone/iPad first. Don't forget to set it back to On when you are through and also set Send Last Location to ON.
Connect your device to your computer. Then import all your photos from the device to the computer and copy all important documents directly to your computer as well. Then do a full backup to iTunes, just follow the instructions below. Then select Restore iPhone/iPad and iTunes will install a fresh copy of the current iOS and will erase all your data and settings. After iTunes has installed a fresh copy of the iOS, it will ask you if you want to set your device as New or restore from Backup. Choose From Backup. Restoring from backup will bring back all your Settings, Apps, Photos on Camera Roll, etc. You should not lose anything.
I have done this procedure many times in the past and restored from backup with no difficulties - it takes 30-50 minutes.
https://support.apple.com/en-us/HT201252