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. It's easily done by clicking on the "duplication button" on the upper right corner of the ad. An exact copy of the listing will be created.
However, if your listing has the status "archived" or "unpublished" you can easily publish it by clicking "Republish".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. The listing will be send for review and you'll be notified when it has been published.