yoursite.com/wp-admin will take you to the login page (I was just there). After you log in, it will probably take you to another blank page. Once that happens, try entering yoursite.com/wp-admin/options.php in the browser address bar and see if that gives you the dashboard. If it does, try changing to another theme and see if that solves the issue. If you can't get to options.php, use your ftp client or the file manager in your hosts control panel to rename or remove the Oxygen theme folder, and see if the default WorPpress theme will take over.
If all that fails, and you're sure the only thing that has changed is the theme, you may have to restore the old version of the theme from your backup.