Funny things, printers. I was doing battle yesterday - a friend has the same printer as me, it was brought round and I swapped it over. Didn't work so I swapped them back. Now mine won't work.
I had to re-install the software. It seems if you unplug the USB, when you plug it back in, the printer is not recognised.
In short, uninstall and re-install, only connect the USB when it asks.