Go to Seller Admin > View/Edit Products, select the affected product and click Edit to view or change its settings. On the first screen of settings, you can enable/disable Inventory Control if you wish. As you click Next to reach additional screens, you will get to one that has an Item Expiration date (if this has passed, your item would be unavailable) and, if you have enabled Inventory Control, a "Limit product sales to" setting (if this reaches 0, your item would become out of stock). If you change any settings, be sure to click Next until you can Submit changes.