First, make sure you haven't hidden it by accident. Go to the main screen and press the menu key and click Show All. If any icons are hidden that will make them appear.
Failing that, do a battery pull. While the phone is ON remove the battery for 20 seconds then put it back in. Wait for the phone to boot up and see if BBM appears again.
Lastly, you can try re-installing BBM through desktop manager.
And if it's still not there go to options>advanced options>applications and see if
Blackberry Messenger is listed. If it is then something is wrong. You would delete it
and download it over again from AppWorld. Hopefully you have a backup of your
contacts.