By default, Magento will not display products that are out of stock.
If a product has no stock , the website’s visitors will not see it. This is actually not listing all your products, that temporarily out of stock.
Step 1: Log into your Magento Admin Panel
Step 2: Go to System -> Configuration
Step 3: Click on “Inventory” in the left side menu
Step 4: Open the “Stock Options” section
Step 5: Change the “Display Out of Stock Products” to “Yes”
Step 6: Click “Save Config“