everything dead

What is your question?

following the procedure to start the GM3500 diesel pickup, Letting glow plugs warm up and turning key to start position, everything went dead. No dash lights - No nothing. Dead in the water. any ideas? it only occured once and it appeared that when the ignition switch was turned to the start position the result was as if the switch was turned to the off position. Everthing went dead. Everything out. Now no matter what the ignition switch turned to nothing happens. As if it had stayed in the off position. Help please???