Am I sinful after a large amount of semen was discharged while I was sitting in a public place, and then a woman sat in my place? And what is incumbent upon me to do?

If the semen did not go beyond your clothes, then there is no harm in someone sitting in your place. However, if the semen stained the spot where you were sitting, you should have alerted the woman, as it is an unclean spot, and so that nothing from it would get on her. Nevertheless, there is no sin upon you for not alerting her, due to the absence of certain harm.