By allowing your players to visit a plane, you are making it part of your universe.

Any plane of existence can be freely visited by players, provided that:

1. You acknowledge it as existing in your universe.
2. Players have a means of getting to it.

By moving your PCs into the plane of Athas you are effectively taking care of criteria #1. They couldn't have visited it if it didn't exist.

Now that your players know it's a real place that they can visit they can just go there directly using a spell such as Plane Shift. If none of them can cast this spell they might be able to enlist the help of a high level spellcaster who can cast it for them.

Some other means of travel between planes detailed in question What are all the ways a player can get to the Astral Plane? might also be usable here.

This being said you are the DM and can just rule that a plane-eating monster devoured Athas or that a jealous deity blocks access to it.

Edit: It looks like my answer is probably wrong due to how planes are defined in D&D, as pointed out in other answers.