From that short description, I'd guess that your USB cable, your USB port on your computer, or your USB port on your BB have a problem. Diagnoses would of course involve trying alternatives.
Occam's Razor nearly always applies when troubleshooting technology issues!
If anyone has been helpful to you, please show your appreciation by clicking the button inside of their post. Please click here and read, along with the threads to which it links, for helpful information to guide you as you proceed. I always recommend that you treat your BlackBerry like any other computing device, including using a regular backup schedule...click here for an article with instructions.