I think you may need to do a factory data reset. This will return your phone to "factory fresh" settings. Before you do this process make sure you back up any data you want to save (photos, text messages, etc.). Also make sure you have at least 50% battery.
Once you have done the above you go to: Settings > Privacy > Factory Data Reset > Reset Phone > Erase Everything. Your phone will reboot and perform the reset. Your missing icon should be back and most likely other issues you"ve been having will be better too.