Story of people divided by occupation onboard a ship whose subsequent generations have forgotten they are on a ship. Ship is due to arrive at new world and no one is aware.
There are farm people, people who are mechanical, people who are the scientists keeping the ship running. All are unaware of each other. Two farm kids discover a hatch in their "sky" that leads to the mechanical people. They learn they have forgotten the mission and must travel to each section of the ship and convince the people they are on a ship and it will soon arrive at the new planet. I believe there also may be something wrong with the ship but I am not sure.
I read this at least 20 years ago.