Well if you haven't created a lot of content or done much yet, I would start over - at least from that point you'll have a "fresh" start. But if you feel like you need to keep what you have, follow these steps.
1) Get access to the database. You can use PHPMyAdmin to do this. Contact your host if you have questions or need access to PHPMyAdmin.
2) Using PHPMyAdmin locate your WordPress site's database. Inside that table there will be a "wp_options" table. Select it.
3) Use "Browse" to see the data inside the wp_options table. Inside this table are two rows we'll be editing - "siteurl" and "home". Click in the Option Value column to change the values for these two rows back to the correct domain/url where your site sits.
4) Now point your browser back at the URL where your site sits and you should see your WordPress site again.