Yes, that is why we have two type rated qualified pilots at the controls. The procedure would be to declare an emergency and land at the nearest suitable airport. Much the same as if you lost one of two engines. If there is another company pilot on board, they might very well be asked to assist. Another captain travelling on board who was fit to do so, could also assume command of the aircraft in such circumstances.
Sorry but whatever Hollywood might lead you to believe, PPL's et al would not be drafted in to assist. Cabin crew would almost certainly be required to attend to the incapacitated crewmember and to provide a liason to the cabin generally. As for reading checklists, no. The remaining pilot would read them themselves, they should know them anyway, and are much less likely to omit important items.