My 4S also was unable to detect wifi networks. I heated my phone with a hairdryer and the wifi settings open and at a certain point networks were detected. I thought I was fixed but the next morning wifi was gone again. So I did the same thing but this time after networks were detected I did a hard reset (power and home buttons together) and this seems to have done the trick. I think that whatever heat does to reset the wifi chip, it's probably overkill and possibly damaging to the phone to heat it beyond this point (i.e. till it shuts down). Likewise, the rapid expansion and contraction caused by heating it and putting it in the freezer seems likely to cause connection problems inside the phone, due to damaging solder joints.
I was surprised that your phone didn't detect any wifi even after all of that. Then I found this and figured that your phone's wifi chip might have died soon after you upgraded to iOS 7. I can only confirm this if you are using iOS 7. If that is not the case, then you should try using wifi with airplane mode on or turn of cellular data.
-You aren't the only one with this problem. Take a look at this link:
Heat the phone at speaker end with hairdryer till message comes up for overheating,power down,put in fridge for 15 min to cool.power up and all should work. It did for me.
There is a guide here to fix a greyed out WIFI and searching problem. Basically you have to reflow the wifi IC with a heat gun but you have to be very careful not to destroy the surrounding modules. I did it by covering everything surrounding the wifi IC with thermal grease. It worked for me
