A couple of things to check:

1. Store Status - Is your store set to Showcase Mode? If yes, the Quantity Box and Add to Cart buttons are replaced with a Contact Us link. For more details, see Setting the Store Status in Zen Cart.
2. Customer Shop Status - Is you store set to Showroom Mode? If yes, the Quantity Box and Add to Cart buttons are replaced with either a Login for Price link or Showroom Only message. For more details, see Setting the Customer Shop Status in Zen Cart.
3. Product Type - Is the Product Type set to allow Add to Cart? Check your setting in Admin > Catalog > Product Types. Ensure the checkbox next to This Product can be added to cart: is ticked.