Auto generator of a customizable and designed sitemap page.
Check the page ID of the sitemap page. Check the system code that you pasted.(Surely used HTML mode?)
When the sitemap is still not displayed, other plugins might be conflicts. de-activate the other plugins and check the sitemap page. (If there are conflicted plugins, please tell us.)
The CSS of using theme may be interfere in the selected style. Select the style "no style" and write CSS yourself or use another themes. Or, if you tell us the theme and your selected style, we might help you.