Whenever a new iteration of i OS comes out, we recommend giving it a few days just to let the clamour die down and any bugs get ironed out before you put it on your device.
Then reopen Settings and try downloading the software again.
If your i Phone stops responding during the download it may be that it has hung.
If i OS 11 is there in Software Update but your i Phone or i Pad just won't download it, or it seems to be hanging we advise force quitting the Setting App and trying to start the process again.
To do so, double press the Home Button so that the app switcher appears and swipe up on the Settings app.
While it's worth waiting a while longer, just in case it's just taking a long time, if you are convinced that the phone has crashed then you could reset it.