If you're not able to display any gallery on the website, please make sure that you upload the images to the page where you want to display the gallery. You can't use images that are uploaded with the media uploader or in other pages. They will simply not appear.
You actually have to click the Add Media button in the page where you want the gallery, upload the images (so that they are attached to the page) and only then use them inside a gallery, but only in that page. They won't work in another page.