Here it is. My i-Phone WiFi selection went grey and not selectable. In short WiFi stop working. After spending numerous days resetting Networks, resetting entire i-Phone and following official instructions from Apple Discussion Board, and nothing worked. The i-Phone WiFi was dead.

What worked for me is not for a faint of heart. I found this instruction on the web and thought it was a prank.

1) Take a blow drier and turn the heat on.

2) Take your i-Phone and aim the heat towards the speaker ear piece. Continue to blast heat until you can't hold the phone in your hands any longer and the message comes up "The device needs to be cooled off before it can be used".

Turn the phone off! You probably will need to hold it in the towel by that time.

3) Put the turned off phone in the fridge and forget about it for about half an hour.

4). Turn the phone on. In my case the WiFi came back ON. 

I can't vouch that it will work on your phone, as it did on mine, but if you are about to put it in the trash, it is worth a shot.

Here are the original instructions that worked for me.