BlackBerry Hub is the place where you manage all your messages and notifications on a BlackBerry 10 smartphone. I have had a few odd occasions in which it became unresponsive. Instead of restarting the phone, you can restart BlackBerry Hub by doing the following:
Swipe down diagonally from the top right corner of the display (see the blue arrow in the image below) three times and the Hub will restart.
Some users have reported that it restarts after five swipes. I am guessing it has to do with OS version. Three swipes works on my device running BB 10.3.