If you need to bring your site offline for maintenance, you can do this in the site settings.
First, go to Global Configuration. This can be accessed on left-side menu, or by going to System --> Global Configuration.
Next, on the Site tab, set Site Offline to Yes.
You can customize the message users will see when they go to your site.
Finally, Save and Close.
When users go to your website, they will be greeted by the message you set, and prompted to log in. Only users with administrator access will be able to access your website while it is under maintenance.