The near loss of Apollo 13 in April 1970 saw the mission's landing site and surface activities were reassigned to Apollo 14. NASA also canceled its fourth H and J-missions - what would have been Apollos 15 and 19. Apollo 15 was upgraded to a J mission and Apollos 18 and 19 were flat out cancelled. Apollo 17 became the end of the Apollo program.
It was bad, but things could have been worse. In August 1971, President Nixon proposed canceling Apollos 16 and 17 - his reason, at least publicly, was that Apollo 15 had been such a success why not move forward with Skylab and the Space Shuttle right away? Nixon's advisors persuaded him to let Apollo run its course.
Had the Apollo program gone all the way though to Apollo 20, crews might have explored the Marius Hills volcanic domes as well as the Copernicus, Tycho, and Censorinus craters - these were all planned landing areas under NASA original plan.
As for the crews, some patient astronauts would have finally left the Earth. Normal crew rotation, which saw a backup crew serve as prime crew three missions later, sheds some light on who would have flown when. Apollo 18 would have seen Dick Gordon, Apollo 12′s command module pilot (CMP), fly to the Moon again as commander along with Vance Brand as CMP and Jack Schmitt as Lunar Module Pilot (LMP). Apollo 13′s LMP Fred Haise would have commanded Apollo 19 with Bill Pogue as CMP and Gerry Carr as LMP. And Apollo 20 would have Pete Conrad in command of his second lunar mission with Paul Weitz as CMP and Jack Lousma as LMP.