If you see the installation screen, it means the table prefix is wrong, because it means WordPress tried to find the database tables but couldn’t, so it offers to reinstall WordPress. And seeing how you want to import an existing database, you don’t want WordPress to create a new set of tables for you.
Next time, could you please explain what you see?
I checked your site and for me it loads a blank page with a successful status code. This usually means that WordPress is configured to use a certain theme, but the theme files are missing.
But that’s not a big deal, because now you should be able to add /wp-admin
to the end of your URL, and login to the WordPress admin area. From there, you can change the theme, or reinstall your old theme, reinstall plugins, etc.