Currently Being ModeratedJan 10, 2013 3:24 PM (in response to RH1)
You may have accidentally turned notifications off. Go to Notification Centre on the side and see if the Icon is greyed out.
If that is the case just click it again to turn it on.
If that's on then go to System Prefs and go to Notifications in the top right corner.
Make sure the apps you want have Badges (The little red circle with a number), Banners and whatever else you want on. This should bring back Notifications for you.
Hope this helps,