I was installing the 4.5.0.81 software and it removed all of my applications/programs, then got this error message on my Desk Top Manager "Unable to reconnect to the device during a multi load opperation" Now the BB has the dreaded 507 error screen.
I just had this issue with another BB of mine. What I did was re downloaded the OS then connect to the DM it should recognize your device. If it doesn't a box will usually pop up and ask what PIN and USB you want. Just choose your PIN then hopefully it will find your device. After it does it will probably tell you that the OS needs to be updated. Select update now and after it finishes (allow it to reconnect to device after update install) and then you should be fine.