Well, my daughter came to me the other day with her white iPhone 4S claiming that the WiFi was inactive. Upon inspection, I saw that the button was indeed greyed out. I researched it online and tried several reset options - none worked.
After furthering my investigations, I found all the posts about taking a hair dryer to the iPhone to reflow the solder joint of the WiFi module. A plausible solution.
So I took two Mr. Clean Magic Eraser pads - which were originally developed as super-duper insulators - and blocked off the battery area to protect it from the heat. I took a standard non-ionizing hair dryer and held it about 6-8 inches away from the phone's camera area and began to heat it up. I did this for two minutes. I let it cool a minute, then powered on the phone. No dice.
Not giving up, I decided to up the ante. I layed the phone face down on a wooden bar stool and held my Millwaukee heat gun about 10-12 inches away from the phone and blasted it! I left off the sponges this time and really heated it up. Did this for 3:00 mins. When I was done, the phone was radiating heat and was super hot to the touch - I couldn't even pick it up. I then relocated the phone to a granite counter-top to cool. Of course, my daughter was impatient and turned the phone on and iOS reported that the phone was over temperature and then shut down. I let the phone cool completely and then powered-up the phone. Still no dice.
So, I waited a couple of minutes. Then did the following steps as outlined by BBohio2013:
1. turn Airplane mode ON
2. turn phone OFF, wait 10 minutes.
3. power up phone, turn airplane mode OFF
And presto! Wifi active again! So I don't know if it was the heat gun activity or the above tasks that fixed it. Of course, I'd try the steps above before cooking the phone as I did. So far so good.
Thought I would share.