First, while on your homescreen press the menu key and select Show All Icons if it's an option.
If that doesn't do the trick, then plug your phone into your PC and open BlackBerry Desktop Software. It's at www.blackberry.com/desktop if you don't have it installed on your PC. Once your phone has been recognized, click Applications on the left panel. You should be able to reload it from there.
Being a core application, you might not have the option to load it that way. If that's the case, once your phone has been recognized, click Device at the top of the software then Restore. Pick the most recent restore point and proceed. If you haven't done any backups and Restore is not an option, then click Update and when you're told your phone is up to date, click View Other Versions to reload the most recent OS, including your SMS icon.
I hope that helps you. 
