This issue can occur if inventory tracking is set on the product, but no corresponding inventory is set.  To check:
  1. Search for the product and select edit.
  2. Go to step 14 "Manage Inventory".
  3. If "Yes, track inventory" is checked and no quantity is set on the third step it will produce this behavior.
Either uncheck the track inventory option, or add quantity to the available/on-hand section.  Once completed clear browser cache, or try another browser that hasn't been used to access the store.