Under the "My listings" tab, you'll see the status of your ads.
If it says "Rented out" you'll have to make a new ad since this can not be reactivated.
However, if your ad has the status "archived" or "unpublished" you can easily publish it by clicking "edit". Make sure all information (rent, date of expiration, expiration date) is updated, then click "Save and Review." You can now preview your ad, double-click everything and then click "Publish Now" at the bottom of the screen.