Since you mentioned that you have modified the theme accidentally, you can re-install a fresh copy of the theme.
As for the demo content, you can only import it once via Theme Options-> General-> Import Demo Content . Do not do it more than once. The following is the reason why. (Copied from the popup when you click the import button.)
WARNING: Clicking this button will replace your current theme options, sliders and widgets. It can also take a minute to complete. Importing data is recommended on fresh installs only once. Importing on sites with content or importing twice will duplicate menus, pages and all posts.
There is no way to reset the whole system via Theme Options or WordPress admin.