So your iPhone / iPad is freaking out and you don't know what to do! Don't panic. Many issues can be solved with a good old “hard restart.” First though,
try quitting recent apps by either double-clicking the home button (iPhone) or slow-swiping up from the bottom (iPad),
and swipe up on the recent apps which may be causing the issue. Then reopen the app you were in when the problem occurred.
Still misbehaving? Restart.
Hold down the Power button (sleep/wake) until it says Slide to Power Off, do that, leave it off 10 seconds, then press and hold the Power button again until the apple logo appears and it restarts.
Still bad, or screen frozen so you can't slide to shut down? Time for a “hard restart.”
Hard Restart: Press the Power Button and the Home Button at the same time for at least 10 seconds, until the phone powers off and comes back on with the Apple logo. Once you see the Apple logo, let go of the buttons.
If you are STILL having problems, do a google search for your device plus the specific issue. IE “Airdrop iPhone SE not seeing computer.” If you don't find any fixes on there, make sure your iOS device is backed up, transfer any photos to your computer or another device, and then Restore it via iTunes or iCloud. WARNING! Your data will be lost if it is not backed up! You will need to know your Apple ID password as well because it will ask you for it before allowing you to set up the iOS device again. This is a last resort.