Order no longer on the System

I visited the Tongaat McDonalds Yesterday and I placed an order via the display order screen. I did my order and received my slip for payment. I did forget my wallet in the car. I went to bring my wallet and pay and when I gave my slip to the cashier. The cashier said that my order is no longer on the system. I then asked for the Manager to explain to me why I cannot pay for my order he also said that the order is no longer on the system. I had to leave empty handed out of the McDonalds store without a meal for my family. This is such disgusting service from staff and poor Management at that branch. I would like to speak to the Area Manager to explain to me why my order was not available.