This is usually major hardware error of WiFi Chip. Try Settings App > General > Reset > Reset Network Settings. See if fixed. If still problem, Restore iPhone with current iTunes on computer. If still problem, and if there is Warranty or AppleCare make Genius reservation or set up Service and take or send iPhone to Apple for resolution.
What i did is
turn off bluetooth
turn of all cellar data
go to reset network settings
this should work after a couple of tries
now i cannot connect to my network it won't even find it
i hate ios6 i got my phone as a replacement from apple after i dropped my phone by accident in water i never had a problem with my first phone. This replacment phone from apple is crap ever since i got it things would freeze, my phone would randomly turn off my camera actually wouldnt conncet so i couldnt use my phone in till i took a screenshot of it not working then it worked but next minute wifis broken. so sick of this and they wont me to pay $220 cause its not under warrenty any more when ever since i got the replacment nothing has worked right.
I had the same problem with greyed-out WiFi and then occasionally it would come back on but could not connect. This happened exactly 15 days after the iPhone 4S 6.0.1 went off warranty. I'm in Bangkok, so I took it back to my mobile service provider, True Mobile, where I bought it. There's no Apple Store in Thailand. True said there's no fix from Apple. I had to buy another iPhone 4S at US$274 with only a 90-day warranty. True said it's a hardware problem that Apple is well aware off because, at this one True repair location alone, they are replacing dozens of iPhone 4Ss and iPhone 5s every week. They gave me a receipt with the reason I had to buy a new phone in case Apple should issue a recall in the near future. Two thoughts for Apple: The problem could have something to do with the retina screen because this greyed-out Wifi issue is not happening to iPhone 4S or 3GS. And, checking the complaints about this issue on the Internet, most seem to be coming from people who are in very hot humid places like Thailand and Brazil. I hope Apple will resolve this issue soon, or my next phone will be Samsung's Galaxy Note 2. No wonder Apple stock is being flushed down the toilet. Steve rise from the dead and save us from Apple.
Sin Tax wrote:
The problem could have something to do with the retina screen because this greyed-out Wifi issue is not happening to iPhone 4S or 3GS.
This problem has been reported with every iPhone model ever sold, including the 4S and 3GS (and also the 3G and original). Sometimes it is a software problem, and can be corrected as described here: http://support.apple.com/kb/TS1559.
If this doesn't help it is most likely a failed RF chip. The same chip supports both WiFi and BlueTooth, so one symptom of a chip failure is if both WiFi and BT are greyed out.
There is a rather bizzare fix for the hardware problem, also; put the phone in a sealed plastic bag (to keep moisture out) and put in a freezer for 15 minutes. Sometimes it works.