Ouch! It’s a really bad thing! Well, backup is the first thing you have to do when you’re thinking to update. But now you can try turn off your iPhone and plug to a PC or Mac and pray to turn on. If nothing happens you can also try holding the both buttons at the same time for a few seconds, when appear something you turn off and connect into a PC or Mac again, if anything happens…well, then you must to restore your iPhone and consequently you’ll loose all your files.
I hope I’ve helped! After tell me what you did to solve this problem ;)