Make sure that you are signed into the same iTunes account that you were when you initially downloaded the app(s). Then select 'Account' from the user menu (top-right), and click "Purchase History." Every downloaded app, paid and free, should be listed there. It's possible there could be some latency between when you purchase an app and when it is recorded in your iTunes account, but I've never heard of downloaded apps not appearing here within a day or so.