The Starlost: The Beginning.

Three farmers in Cypress Corners discover that farm life is not so simple. Discovering a hidden door in the earth, they break through it, discovering they are not on Earth, but in a giant Noah's Ark-like spaceship. The spaceship is made up of giant "domes" holding the various cultures of humanity. Eventually, they find the bridge, its crew dead and its computer complaining that the ship is off-course, heading into the path of a distant sun. The three farmers take it upon themselves to travel the ship, contacting the various cultures of humanity in search of somebody who can correct the course of the ship and save mankind.