A simple plugin for WordPress that adds an unobtrusive "back to top" link to your site or blog.
Check out the plugin's page
It could be that your theme doesn't call wp_footer() (as it should). This is a pretty essential part of a WordPress theme, so we recommend you contact the theme's designer and ask them to add it. Optionally, if you're comfortable editing your theme file, you can add it yourself.
This plugin is released under a GPL license.