Try the troubleshooting suggestions in http://support.apple.com/kb/TS1559 and if that does not help, a user has found that it helps to:
Go to Settings and turn Airplane Mode and Do Not Disturb ON
Power down and wait 5-10 minutes
Go to Settings and turn Airplane Mode and Do Not Disturb OFF
If that does not help, backup your content and go to Settings > General > Reset > Erase All Content and Settings and then restore from the backup.
If that still does not help, take it to an Apple store genius bar for testing by the technicians.
After Apple's recomendation (and also the freeze and the hair drier) I tried Ralph's method too, but my wi-fi it's still grayed. It's very confuse, some cases seems to be hardware and in others something of software. Mine maybe its hardware because the freeze works by a couple of days. I don't know... and I don't have warranty at this time in my 4S.. By the way... Thanks.
I finally manage to bring my iPhone 4S to a local repair shop since it's out of warranty (1.5 years since the purchase date).
They're not cheap (it costs me ~$65), and apparently they're professional in their job (well, that comes with a price, doesn't it?..)
I'm not quite sure what exactly they're doing, but looking at how they use airgun, and dismantling the iPhone, most likely they're reflowing the wifi chip, as the solder got cold and didn't make a good contact between the chip and the board.
If you browse the internet, you'll find someone suggest to do reflow on the logic board or put in the oven for a certain amount of time.
At least I can keep my iPhone (with wifi functionality) a bit longer before upgrading to a newer generation smartphone
This is clearly not an Apple shop, and questionable about it being Apple Authorized Service Provider...and what you describe is certainly strange. With the way logic boards are produced, you cannot get a few cold solder connections. They will all be good or all bad. It will be interesting to hear the results and if the iPhone works at all after he repair.
You're correct it's not Apple shop but it doesn't mean that they're all bad. Mine so far working about a week. Maybe I should post again in another 3 or 6 month.
Even for me it's hard to believe about the cold solder join at the beginning.
I search the net, and it seems people have more complain about this on January 2013 onwards.
Some theories flying around the internet. Some say that iOS6 uses different kind of checking on wifi chip during startup, so that when it didn't pass the check it will not allow us to turn it on. Mine began to be not working after upgrading to iOS6 (iOS 6.1 at that time, February 2013, exactly 1 year after the purchasing date, which makes it out of warranty).
Some say it's a combination of software and hardware issue.
Out of some other theories, only Apple knows the truth
I do electronics repair for a living. But I don't use the air heat-gun, since I just work with larger size of chips where soldering iron still works fine. If I had the proper air heat-gun, I would have done it by myself.
If you say all be good or all bad, the iPhone production line itself might have set the wifi chip soldering in its tolerance line, so there's a higher chance that it will fail. It's just another theory of mine.
stupid Apple tech said my warentee is done with them I had to pay 200 for this prob my wifi is greyed out cant use it I just read what was posted and did what was suggested hope this works thanks Ralph I am dealing with a rental company my 1st i phone froze on me and lost my calls now on the replacement one wi fi wont work or my B tooth
How is the Apple technician 'stupid'? Your phone is out of warranty. That is not the technician's fault.
Did you choose to extend your warranty?
Did you choose to purchase some form of protection plan? (there are MANY sources for this; your carrier, Apple, your credit card compnay, etc.)
In short, have you taken any personal responsibility?
You mentioned $200. Is that the out of warranty replacement cost? Depending on what version of the iPhone 4S you have, you just saved $349 to $549, because of a program that Apple offers for out of warranty service, which is not required at all.
Sounds to me that you got off lucky, especially since you didn't take any steps to protect your personal property. Do car manufacturers let you buy a replacement vehicle at less than half price if your car is out of warranty and you had no service plan? Nope.
Think about what actually happened, and realize that it could have been a lot worse.