Few reason that may cause a WordPress site become blank page.
1) Changes of path name, I.e wp-content & wp-includes
2) Error of configuration which cause disconnection between index.php and wp-content or wp-includes.
3) Wrong coding which effect the WordPress root coding, usually if there's wrong coding in theme file, it will appear error notice instead of white screen. So I guess u have change something in root folder or wp-includes.
There's no restore button (or maybe it does, but I can't find). The only way is backup your WordPress database (I.e using phpmyadmin to export the wp database). Then
Remove and reinstall your WordPress. If you're using auto database creation. Back to the new WordPress database, remove all the existing database table (make it into empty) and import your back up database.
Your existing settings, post and page should be restored. But you need to find back the theme. If your theme is working fine, use FTP to make a backup copy of your theme before you remove the faulty WordPress. After reinstall, transfer the backup theme back to your wp-content/theme . Back to appearance and active
Hope this is clear and help!