The green loop that you pull up to make the mower drive forward has 2 functions, it activates the clutch and it turns the motor on. The motor is turned on by a switch inside a black box on the left side of the mower handle, the green loop handle rotates a central part which in turn presses a plunger on a switch. The left side of the mower is on your left as you push the mower in normal operation. Sometimes the green loop handle can pop out of the central part so it is not being rotated, please check this first.
The image below is what the inside of the left switch box looks like. The switch marked 10081631 in this image may have stuck down or be faulty, some WD40 or light oil may help it to move freely, if not then we can send another switch which is very easy to fit, it only requires the 2 electrical spade connectors to be removed then the switch lifts out. Please disconnect the battery whenever this product is open.