I phone wi fi button won't switch on
I phone 4 wi fi button won't slide across to switch wi fi on
Start with the troubleshooting steps in http://support.apple.com/kb/TS1559 and if that does not work,
Settings > Airplane Mode ON and Do Not Disturb ON
Power down and wait 5-10 minutes
Power up
Settings > Airplane Mode OFF and Do Not Disturb OFF
And if that does not help, backup your content and reset to factory condition and restore the backup content.