No Items Visible?
Edit Page -> Advanced Settings Tab
1. Under Checkout Options, make sure the the option to Allow Payments (Donations) to be accepted is checked. This is usually the key setting.
2. Under Checkout Options, make sure the the option to Allow anyone to who comes to the page to pay is checked.
Edit Page -> Collect Options Tab
1. Make sure that all items are not expired (advanced tab in item)
2. Make sure that all items have qty left (advanced tab in item)
Edit Page -> Personalize Tab
1. Make sure the deadline hasn't passed to pay.