I have a 2003 jaguar x type 3.0 and I was recently at a stop light when it turned green I went to go the car was slow to accelerate and immediately the gearbox fault came on the dash board...need help

Auto trans? On some cars there is something called a tcc, or torque converter control solenoid. It locks the trans in higher gears for cruising and helps you get better fuel mileage. Google to see if your car has it, if it does, unplug the wiring to it and see if it "fixes". If so, the solenoid is bad in the granny. It won't hurt to drive this way, but you will lose fuel mileage.