Your post is pretty vague, not too sure what happened to cause this. Did the USB cable disconnect? Are you using a USB hub?
Reboot your PC and the device, verify your USB cord is connected to the back of your PC, not a monitor port or a USB hub.
Then follow these steps to the letter: What to do next if Jl Cmder will not connect and wipe - BlackBerryFAQ
Be patient as it may take multiple attempts before it connects.