The a look at the following, #2 KB23968-BlackBerry Messenger icon disappears from the Home screen of the BlackBerry smartphone
Other than that, if the OS version isn't the problem, I would try first a security wipe. If that doesn't fix it, I would reinstall the OS. Do a backup before you do anything.
Worst case, you will have an opportunity stick with the old version of BBM that is in the OS package.