Have you got WiFi Switcher installed? If so it means you're WLAN is disabled via that app. Tap the status bar, then tap the WiFi Switcher button. You should get a list of WiFi access points after a few seconds. Now if you click outside of the list, you'll be returned to the desktop and that icon will have disappeared. If you repeat the above process, instead of getting the list of WiFi APs, you'll get a notification that WLAN has been disabled and the icon will reappear.

If you don't have WiFi Switcher, then it just means your WLAN is not available for some reason. In either case restarting the phone should clear it.