My daughter needs help. She was updating her iPhone through her (MacBook Pro) computer. After hours of updating the update failed and her screen (I think it's her computer screen) says this, "restore phone to factory settings and erase phone". She is devastated that she'll lose all her pictures, videos, etc. Are you able to help her so that she won't lose anything? I think she was saying her phone won't turn on.
I ran into this problem at work and the end user ended up spending a fair amount of time on the phone with Apple support getting out of this mess.
A phone call to Apple support might do it but the suggestion to make a genius appointment is probably the best idea.
I do hope you made a backup of the phone before proceeding with the upgrade. If you did go ahead and restore the phone as new and then restore from the backup.
A small bit of advice. I have found thru personal experience that upgrading iPhones via iTunes is much slower (and less reliable) than doing it right on the iPhone. I don't know why this is but I never upgrade the O/S on my iPhones via iTunes anymore for just this reason. go into Setting -> General -> software update and do it there with the phone plugged into it's power adapter. Also I always make a backup of the phone before each and every O/S upgrade. See, I learned the hard way...