PixalZA is correct. I was about to respond. That setting should be turned on by default so that implies you have somehow managed to turn it off. What else have you turned off or on?
Be mindful of arbitrarily pressing buttons in maya there are lots of hotkeys and it is really easy to flip things on and off that will quickly drive you to insanity trying to figure out how to undo!
If you ever have a situation like this the first thing to do is rename (do not delete, just rename your maya folder) - be sure to close maya first.
Then reopen Maya this will reset all the default preference settings. See if this fixes your problem then decide is it easier to figure out what hotkey you pressed and fix it or just start from a fresh prefs.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675
Last edited by ctbram; 03-07-2012 at 10:15 PM.