I had an issue with a creative zen. Try the following: Go to Control-panel-device manager.
Click on 'portable devices' and the zen should be listed. It may well have a warning sign on.
Right click and click 'Update driver software'. Click 'Browse my computer for driver software'. Then click 'Let me pick from a list of device drivers on my computer. There should be an option of 'mtp device' Click that and then click 'next'. That should reinstall the driver and your pc then pick up the zen device again.