This has only happened once or twice atmy parish because we have more than one priest and they live at the rectory on church grounds. If there is not priest even 5-10 minutes before Mass is to begin, someone can go over to the rectory and alert the priests. I know on one occasion the priests had swaped Masses and someone just forgot.
If the parish only has one priest and he isn’t there at Mass time, there should be a process to follow to find him or another priest. i.e., first alert the parish office, then page/text the priest, then call neighborhing parish X. If a priest is late or missing for daily Mass, it would probably just get cancelled. For Sunday Mass it would depend on if the priest would be avialable for the next regularly scheduled Mass, then people could just come to Mass then. If there is only one Mass, people can choose to go to another parish if possible, the parish can have an emergency process for “Sunday service without a Priest” (Communion Service) as another poster mentioned, or people just don’t go to Mass.
If there is no priest and no Mass is available, no obligation exists for that Sunday.