I'm assuming you have tried unplugging and re-plugging it back in again? What about different USB sockets? Can you try a different cable? The only time this has happened to me, a reboot is all that was needed.
I was going to suggest that there may be a "USB mode" which the camera needs to be activated - but I suppose you would already have been aware of that. Try resetting the computer and plugging back in again.
Asus Eee 701 Netbook - 2GB OCZ RAM
Debian Edition - Xfce