If it was a fair fight and fairly clearly an accident (for example a lance tip through an eyeslit), it's unlikely anything would happen. Everyone knew the joust or the grand melée could be dangerous; that was part of the attraction for the medieval warrior class. Injury or death was a risk they knowingly took. If, however, the Lord was killed with clear malice aforethought, for instance if several knights were seen to deliberately target him during the melée, the perps could potentially be challenged by one of their victim's comrades or superior to trial by combat, either to first blood or to the death. In medieval Europe, at least, they would not be being brought to trial in front of a judge.