After updating to WordPress 3.8, the single-column Dashboard disappears with no options to bring it back. For years, users could set the number of columns in the Dashboard to suit their needs, but apparently some brilliant decision was made to just remove it. Personal opinions and feelings aside, here is a quick snippet to bring back single-column Dashboard layout for those who were using it and wish to continue doing so.
There are many ways to customize the WordPress Dashboard. Over the years, the Dashboard has evolved into a highly flexible information portal, enabling an overall, big-picture view of the main components of your site, while also providing granular data on everything from recent comments and plugin updates to incoming links and WordPress news. And that’s just the default functionality, there are also a ton of dashboard widgets and plugins available in the WordPress Plugin Directory that you can use to transform your Dashboard into just about anything, or even disable it completely.