For many people, this is not a fix and the advice given isn't correct.
I foolishly suggested that my client upgrade her 5S yesterday.
She ended up with the frozen "slide to upgrade" screen.
I plugged the phone in to iTunes. Selected "back up now". It pretended to do something. I then selected restore. It told me that I need to go to settings in the phone and turn off "find my phone" (duh...).
I deleted the phone from find my phone in icloud.com but still...