What can cause the no spark problem on my Ford?

I have a 1993 Ford Explorer. My sister was driving down a road when the engine stalled. I hooked up a spark tester and the plugs has no spark. It has no distributor. But it has a ignition coil. I brought the ignition coil to my auto shop and the teacher said its still good. The spark plug wires are still good. The spark plugs look pretty bad. Can it cause it to have no spark? What else can be the problem?