So my dad recently tried to install iOS 8 on his iPhone 5. We have encountered a problem where the boot will get to about 95%, then go no further. I'm then unable to turn the phone off, or interact with the phone at all, and it would appear that google doesn't have the solution. I'm currently trying to drain the battery so I can try to reboot, but that is going to take a while because I can't do anything taxing on the phone's battery, due to the fact I can't do anything on the phone. In the meantime, I was wondering if any of you guys had some ideas?
If all else fails, you could always just open the phone up and take the battery out yourself...that would save you a lot of time and trouble waiting for it to drain.
Then, once it is off, boot it into recovery mode (http://support.apple.com/kb/HT1808). Then connect it to iTunes and either flash iOS8 onto it again, or alternatively you can get iOS7 from a number of internet sites and flash that back onto it (I am not going to tell you how to do this as it is very specific to your device. You will need to figure out your device model (found on the back of the phone) and the corresponding OS package for that model).
Either way, start with opening up the phone and taking the battery out. iFixIt has a great guide on this.
If the phone still won't boot after you turn it off and put it into recovery mode, then you might have to take it to Apple.
9 times out of 10, a restore works perfectly. I'm not sure that you can do that in this situation however. Other than that, CaleTheKing listed some good ideas. Disconnecting the battery is a little overkill in my opinion, but everything else is fine.