Where is the site hosted? Does your web hosting plan include nightly backups? (If not, you should consider moving to a Managed WordPress host plan that does offer nightly backups.)
If your web hosting plan includes backups, your best bet is to log into your web hosting control panel and restore the site from the most recent backup — before you made the change to the domain name.
If your web hosting plan does not include backups, you can still regain access to your site by editing the database using phpMyAdmin — a tool that should be available within your web hosting control panel — and editing the wp-config.php file for your site.
This article contains instructions on how to change the Site URL yourself:
But if you just want someone else to fix it for you, you can chat with a WordPress expert here, and they can get you back up and running in short order at a low cost.
Hope this helps point you in the right direction!