I found a sparkplug in my 2000 ford taurus that

This is called limp mode, it's intended to let you limp the car home when the transmission fails. If you continued to drive the car with only 2nd gear it can be very hard on the engine to continue to drive at highway speeds. I don't know if that is the case or not. There could be a variety of problems here, I would take it to a shop, the transmission needs some service anyway. Most likely it will require a rebuid.