There's been a lot of chat about iPhones crashing thanks to Apple's latest update. People have been saying that their phone is bricked. Thanks stupid Apple.
The problem is the Slide to Upgrade screen isn't responding, so here's some thing you can do to fix everything. Kinda.
Apple have said that they're going to release a new update soon, which will sort everything out, but if you're in a rush and can't wait for Apple to pull their finger out, here's some workarounds.
What To Do If Your iPhone Won't Work After Updating
You need to connect your iOS device to a computer that you've previously synced with iTunes and make sure that iTunes is open.
Then, you need to select your device and if you don't see it listed, you'll have to do a 'Force Restart'. You do that by pressing and holding the Sleep/Wake button until the red slider appears. You drag the slider until your device turns off, and then, when it has done that, press and hold the Sleep/Wake button again until you see the Apple logo.
If you did a backup of your device before updating to iOS 9, then restore it to the previous OS. If you don't know how to do a backup of an Apple device, click here to see how. Hopefully, that should do it.
If not, and you're still seeing the Slide to Upgrade screen and everything is borked, then you'll need to restore your device.
If you don't see your phone on your computer, press and hold both the Sleep/Wake and Home button for 10 seconds or so, until your device restarts and appears in iTunes. Then, it'll prompt you and you'll choose Restore. Basically, this is a factory reset, so your phone will be as good as new, but you might lose some stuff if you haven't backed things up or saved them to a cloud.
From now, thanks to Apple being a regular glitchfest, it'd be worth doing sporadic backups of everything, just in case this all happens again/