I have seen this a few times with other updates. There is no good answer for it beside it occasionally happens. Try pressing and holding both the power and home buttons for about 15 seconds and you should see the phone screen blink and possibly the apple symbol appear. Depending on where it was in the update you could see a few things:
1) the update picks up where it left off and continues successfully (YAY)
2) The update was interrupted and you will see the iTunes symbol with a plug indicating you need to plug it in and restore (this is not fun but hopefully you backed up before updating)
3) the update was not very far along and got interrupted and the reboot causes it to cancel all together in which case you are returned to your iPhone before the update and you can try again (if this is the case i suggest using iTunes to do the update)
Let me know how it goes. I support a few hundred Apple devices for work and tried to put these in order of how many times i have seen each one. Scenario 2 i have seen more frequently lately but i have by far seen scenario 1 happen more.
Good Luck!
iPhone 5S, iOS 8.2
iPad w/ Retina, iOS 8.2