my iphone 4s wifi button is grey why,

In order"

Work through the tips in http://support.apple.com/kb/TS1559

Try rebooting the iPhone by holding both the power and home buttons until the apple logo appears and it restarts, ignoring the red slider if that appears.

Settings > Airplane Mode ON, Do Not Disturb ON

Power down and wait 10 minutes

Power up

Settings > Airplane Mode OFF, Do Not Disturb OFF

This method don't works for me..

Then bring your phone to Apple.