There's quite a comprehensive guide to this covered by ASchroder:
Why are my Magento products not showing up?!
Check your store's default configuration for stock management via:
Stock Options section, you'll see the setting for
Display Out of Stock Products, which in your case is likely set to no.
Product Stock Options, you are able to see your default setting for Manage Stock & Qty for Item's Status to Become Out of Stock.
I'd assume that due to the products being stock managed & out of stock, they're no longer visible on the frontend.
Remember to refresh your caches & re-index after changing configuration & stock options to ensure that the updates are visible on the frontend.