One thin layer, then nothing. Z problem?

What would cause a printer to lay one layer, then stop extrusion? The the print head doesn't move up, and doesn't push any more material, it just traces paths right on the first layer.

Has it worked before?

Layer height 0?

Z axis defective (motor or drivers)
Z axis not setup properly in printer...

When you ask for help it really helps to give as much information as possible like make / model of printer, settings, new problem or new printer, settings used and most of all photos of the issue.