Under the "My listings" tab, you'll see the status of your listings.
If your listing has the status "rented out" you'll have to make a new listing since this can not be reactivated.
However, if your listing has the status "archived" or "unpublished" you can easily publish it by clicking "Republish". Make sure all information (rent, move-in date, move-out date) is updated, then click "Save and review." You can now preview your listing, double-check everything and then click "Publish now" at the bottom of the screen.
If your thinking about reactivating a listing that was published a long time ago you should instead create a new listing.
When you reactivate a listing you also reactivate all previous applications and conversations to that particular listing.
Soon you will be able to simply duplicate a listing that is either archived or rented out.