In short, save the model to a mat file using the CheckpointPath option, and load the mat file later. Then you can resume the training my passing the layers of the checkpoint during training, as follows
trainNetwork(XTrain,YTrain,net.Layers,options)
Note that if you reuse the same options, you might see that the training diverges, as the training might have reduced the learning rate. Hence, use appropriate options as required.
