If you have the Desktop Manager software on yoiur pc you should be able to plug your berry in and reinstall it with the application loader. Once you connect go to app loader, then make sure BBM is checked and proceed and viola it will be back.
Its probably just hidden. My browser did that to me when I got my blackberry. Hit your menu button to show all your icons and then once move and select show all. Find the messenger icon and menu button and then uncheck hide.
