I had this same problem and was pulling my hair out over it. Things went bad for me when I installed the device software on my computer. After that I could not longer detect the phone even though I could use the Mass Storage Mode.
Here's what I did to resolve me problem, maybe it will help you:
-Connect your BB via USB
-Right click on My computer and select Manage
-Select Device Manager
-Expand the Universal Serial Bus Controllers list
-You should see two entries related to BB
- Blackberry Smartphone (Most likely has an indicator saying that driver is corrupt or not found)
- USB Mass Storage Device
-Right click on both of these and Uninstall (since update driver will fail)
-Safely disconnect BB from computer
-Reconnect BB via USB
-Windows should auto detect new hardware and reinstall the drivers for the BB
-Back in business with Desktop Manager