You are going to need to reload the handheld operating system.

Read this thread - For those of you needing a quick copy of Javaloader... - on how to use "Javaloader", a program to wipe out the BB.

Download and install the latest handheld operating system for your BlackBerry from your carrier.

Wipe out the BB using Javaloader first. Then restart Desktop Manager and it should load the OS.