By allowing your players to visit a plane, you are making it part of your universe.
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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][1]. 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?][2] 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.

  [1]: https://www.dndbeyond.com/spells/plane-shift
  [2]: https://rpg.stackexchange.com/questions/104036/what-are-all-the-ways-a-player-can-get-to-the-astral-plane