Navigate to the Library via the Page Navigation Menus on the left, and select Marketing Items.
Find the Marketing Item you want to deactivate/reactivate, select it, and a popup window will appear. To view Inactive Marketing Items, select the SHOW INACTIVE button on the right hand side of the screen.
Here you will see all the details relating to this Marketing Item. If you look in the bottom left hand corner of the popup window you will see the Active/Inactive slider. Click on it and set it to the option you want to set the current Marketing Item to.