This maintenance message shows when WordPress updating its version, theme or plugin. Sometimes the updating process gets interrupted or fails for any reason. To fix this problem, delete .maintenance file in root directory of your WordPress folder.
That’s all. Simple as it gets.
In case the “. maintenance” file was not there at all. Started exploring the blog files. In wp-content/upgrade/<pluggin_name>.temp. The pluggin name was the latest plugin I tried to update. So I deleted this folder and everything came back to normal state.
Reference: WordPress FAQ