Until few days ago all was fine. Now WIFI is off and won't activate to turn on at all
iOS: Wi-Fi settings grayed out or dim
Follow the article provided by modular747 if the issue remains I'd suggest contacting AppleCare for assisstance or booking an appointment at an Apple Retail Store to have the iPhone evaluated.
99% of the time this turns out to be a hardware failure that requires replacement.
What ever you do, don't try the "freezer" technique you might read about which provides only a temporary fix, if at all, and risks damaging the phone in other ways.
Many times a backup and retore from itunes will fix this. If not then contact Apple.
Retrieving data ...