If you were attempting a site migration you should have backed up all your files and database tables before changing your URL. When you change your site URL (presumably from the Settings->General option page), you are essentially modifying every link on the site to point to the new URL. If that URL is not setup with WordPress on it, then your site will crash wonderfully.
Fortunately, nothing is too permanent in WordPress. If you have access to your database, you’ll want to get in there and revert your website settings back to your existing (and working) URL. The table that holds these entries is the “wp_options” table and it most likely will be the first two entries in there with option names of “siteurl” and “home” that will have your newest/ latest changes to your URL.