This has happened to me a few times with my Iomega 1GB via USB. But it becomes unrecognized after being used and then just sitting a few days, coming in and out of a full PC hibernation. Rebooting the drive fixes it.

Try this for yours. Unplug all USB devices. Go into device manager and delete all the USB ports underneath the USB controller tree. They are called 'USB root hub.'

Reboot the PC and plug in the other USB devices and let it rediscover them. Plug in the Iomega and see if it is rediscovered. If not, does it have a firewire port too -- mine does. Try it.

Otherwise, it may be loose or the box may be gone. Cheap external cases are available everywhere - or mount it inside the PC.