Frozen iPhone 6

A small message window came up on my iPhone 6 that wants me to allow an app to sign in using facebook. It's the kind of window that has two option buttons 'Cancel', or 'Continue'. However, the thing has frozen up on my screen and does not respond to pressing either of the two buttons. I have tried to Power Off the phone, but althogh the Power Off icon comes up at the top of the screen, it too will not respond to the sliding action necessary to power down.

Essentially, the phone will stay unresponsive untill I can get rid of this message box, but how do I get rid of the message box if the phone is unresponsive.

Any ideas will be greatly appreciated.

Cedric.