The cruise control speed can be reprogrammed for any desired speed at any time simply by holding down either of the set speed buttons.
Therefore, the most likely cause for this would be that the cruise control buttons have been held down while the trolley is stationery as doing so will set that button to operate at 0mph.
To sort this, simply use the forward and reverse buttons to get the trolley moving at the desired speed and then press and hold down the required number cruise control button until the trolley stops momentarily. Once the trolley stops release the button and the trolley will start moving again and in future pressing the button will set the trolley at that speed.