1994 Toyota Previa will not trun over.

I have a 94 toyota previa 2wd (non supercharged)with a little over 100k miles. Recently on a short trip the vehicle started shuddering and acting as if not all cylinders were firing. I was able to get the vehicle home despite it being very difficult to start but when I tried to start it again it would not trun over. I removed the spark plugs and wires and let it sit thinking I had perhaps flooded the engine But when I replaced the spark plugs and wires in order to try to restart the vehicle to see which cylinder is not firing the engine made a "thunk" and did not turn over. At this point I am lost. I know that the spark plug wires are in the correct order and there are no warning lights on in the vehicle. I thought this was nothing more than a spark plug gone bad but now I'm not so sure. Any suggestions?