An easy way is to create a file called ‘maintenance’ and place it in your sites root folder. When WP sees that file, it puts up a ‘down for maintenance’ type file. You can still get into the admin area, but the ‘public’ can’t see the file.
Another way is to rename the ‘.htaccess’ file (which helps redirect access to the site into WP pages) and create a new index.php/html file (renaming any existing one). That index file would contain an ‘under construction’ message or something similar. Disadvantage of that would be no access to the admin area.
Which technique you use depends on the length of the downtime. Temporary downtime: I’d use the ‘maintenance’ file technique. Longer-term: I’d build an ‘under construction’ page as index.html or index.php (renaming any existing files of that name) and rename the ‘.htaccess’ file.