You are going to need to wipe out and rebuild the BB.
Locate and download a copy of Javaloader.EXE
Locate and download the latest handheld OS from your carrier. Install the downloaded program.
Connect your BlackBerry and make sure Desktop Manager is shut down.
Open a DOS prompt and type
javaloader -usb wipe (which will completely erase the BB) If you do not have a backup you will loose all the data, but at this stage of the game you dont have a heck of a lot of choice in the matter.
Then restart Desktop Manager. It will detect the BB and reload the OS.
For more details, refer here: BlackBerryOS 4.x Download FAQ - UPGRADE DOWNLOADS