iPhone 4S wifi will not turn on How can I fix the gray wifi icon? It will not slide to turn on wifi, it's been happening since the new ios7 update
that is a trick you may tried it out, it's work for me for now.

1) use air dryer heat up your phone at the top part(you can aim on the headphone jack) until you iphone shown up the temperature screen.

2) When the temperature screen show up , turn off you phone (caution! the phone is very HOT).

3)After it is off pit it into a zip lock bag.

4)Let it rest for 1-2min and then put into the freezer for 30Min

5)When you take it out let it remain in zip lock bag and turn on the phone ( it may show battery low screen. If it's happen wait for a while and try turn it on again)

6) You are done!! you fix your wifi.

It will grayed out again whenever you update to a newer version of ios, so if that happen redo this proccess. So far it work for me, you may give it a try..