iPhone 6 - "Select a Wireless Network" menu has frozen my phone!

I was on way home from work. At a light. I checked phone as I do any other time. The phone began searching for a wireless network (there are many in the area I live in) and displayed the menu "Select a Wireless Network" as it always does - but this time it was empty. No networks available AND (more importantly) it is frozen on the phone display! I cant get this window to cancel or go away. I tried powering down but it wont let me do that until I somehow cancel this window! But I cant do it! WHAT DO I DO? Phone is working normally behind the scenes - I hear when messages come in, I can see other windows grayed out behind the Wireless Network window. Help!! 😟