When a cable becomes unplugged or training is lost for any other reason, the TRAINING_LOST interrupt occurs. At that point, video data and MSA values should not be relied on.
After the cable becomes plugged in again, no action is required from you; the core properly resets itself and applies HPD. In a scenario, where the cable is plugged-in but the training is lost, the software is expected to assert a HPD upon the occurrence of a TRAINING_LOST interrupt, so that the Source can retrain the link.