To reactivate an inactive item, you need to ensure the item has a quantity and price greater than 0, a valid condition, and that the "make active" box is check marked.
You can look up an item in Fillz by clicking on Inventory->Browse, and searching for the item. For help searching for an item on this page, see the "Searching" section of this page. Once you have found the item you would like to reactivate, click on the SKU to get to the Edit Item page. On that page you can update the quantity, and any other necessary information.
Alternately, you can import an inventory file into Fillz to increase the quantity of an inactive listing. Click here for more information about importing files.
Lastly, you can import files via FTP. For information about FTP, click here.