How to get Nikon Speedlight SB-600 flash to work?

I Bought a Nikon Speedlight SB-600 in August. Tested the flash and it fired ok. I took out the flash a few days ago to use at a graduation and the "ready" light did not come on. Consequently it did not flash on any of our nikon cameras nor did the "Test Flash" fire. Help me as I can't afford to buy a new one. I tried several new batteries including some from other photographer's speedlight sb-600 and sb-800 flashes that I know to be working and the flash unit just would not go off.

Added (1). I made sure that the batteries have been inserted properly each time. The flash unit turns on every time but the "ready" light never comes on.

You could have a failed unit. Warranty should cover this.

Other silly thoughts…
Be sure you're putting the batteries in the proper way. There's little diagrams showing the correct orientation of the battery in the battery compartment.

Be sure to press the power on button for a second or so. It won't turn on with a quick press of the power button.