There's been some anecdotal information from the field that the device may sometimes be in airplane mode, but is not displaying the airplane icon as it should, so first check this by pressing and holding the power button > 1 second and seeing if it shows being in airplane mode or not. If it is, change it to be not in airplane mode, then try to turn on WiFi again.
If that's not it, then try shutting the device down and restarting it. This usually clears the issue. Why WiFi is not turning on is due to a driver not loading correctly either because of a hardware race condition, or other issue. If you restart the device and it still won't turn on, send us your serial number and we'll issue you RMA paper work to return it to us.