Hey kiran;
I was having the same problem. After probing around the settings in my LG Optimus L5, I found there's 2 ways of setting notifications. One is through System Settings->Apps->Messaging. In there, you can choose to turn off notifications, Clear Cache, or Clear Data. Doing the last 2 things doesn't seem to help. Then I started my Messaging app, pressed the menu button, clicked on Settings->Notifications. Ah-ha...the messaging notifications had somehow been turned off (this is a different setting that the other 'System Settings'). So I turned them back on, and it works fine now. It must have been a side-effect from something else..possibly an Android bug. Hope that helps.