Well if you have restored it and checked all settings I would assume its a hardware issue. Its possible the WiFi antenna has come unplugged or has gone bad. If its still under warranty then you could try taking it to apple and they may just replace the phone for you.
If not, or an apple store not near by....Then you should open up the iPhone and make sure the antenna is snug and plugged in on the bottom of the motherboard. If you still can pick up any networks, I would order a replacement antenna and replace it. The antenna itself is a cheap part and isn't a difficult replacement.
Hope this helps!