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The normal troubleshooting steps with an iPhone are re-booting the phone by holding down the home and sleep buttons until the Apple logo appears. If that doesn't work connect the phone to iTunes and take a backup. Then set the phone up as a new phone to re-load iOS. If that cures the problem you know it is a software fault.
Restore the backup and if it stops working again you know there is a problem with one of your apps.
If re-building the phone doesn't work at all, it is a hardware fault and it needs a repair.
Have tried setting up as new iphone and wifi was totally greyed out, didnt get the option to search for wifi. Powered down phone and has let search for it now. Havent downloaded any apps yet, just got the basics the now. Will leave and see if wifi disappears again. Not touched it and it has just disappeared after couple of minutes. When you go to wifi the choose network circle is going round and round for ages and not picking up homehub.
I have tried all these a couple of times at least and still having problems with wifi. We have another iphone 4s in the house on exactly the same software version and no problems with it. Iphone 5 fine as are 3 ipads.