You could have too many links in the sitemap for example, causing a timeout or memory limit exceed. Or you could have some bugged plugin installed causing an error with XML documents.

Check the server error log or enabled the debug mode in JSitemap and the error reporting in the Joomla global configuration. In this way you will be able to visualize the exact reason of the error.

Also, if the problem is the number of links, you can just unpublish some data source to reduce them and check if it was that.