If you would, please re-post w/more information regarding your phone model & carrier of the following doesn't answer your question :
I believe you've loaded an application that is a SENSE replacement.
To re-enable SENSE, you'll need to clear the default(s) for the SENSE replacement application:
Go to SETTINGS > APPLICATIONS > & scroll down until you find the application that replaced SENSE. Long press on the app, release & you should see a 'CLEAR DEFAULTS' button. Clear the default & SENSE should be re-enabled once again.
