If for some reason you must take down your site or just a few pages temporarily for maintenance, moving servers, or whatever reason, you need to tell the search engines that this is TEMPORARY. Otherwise, they will see the page is not there (404 status/response code) and drop your site or page from their search index.

This would obviously not be good.

You tell the search engine that the page is temporarily unavailable using the 503 status code in the http header. In php, you can set the header for the status code like:

header('HTTP/1.1 503 Service Temporarily Unavailable');
header('Status: 503 Service Temporarily Unavailable');
header('Retry-After: 7200'); // in seconds

In drupal, you can use the equivalent, drupal_set_header function.

You can use .htaccess rules in combinations with a php file to use the 503 status code by:

1) Create a 503 php code page (e.g. error503.php) and put the header code in it like:

header('HTTP/1.1 503 Service Temporarily Unavailable');
header('Status: 503 Service Temporarily Unavailable');
header('Retry-After: 7200'); // in seconds
print "This page is temporarily unavailable";

2) In the .htaccess file, add in:

RewriteRule .* /[path-to-file]/error503.php

(This will redirect all pages to your error503.php file.)

Or you can only redirect specific pages to the file like:

RewriteRule page.html /[path-to-file]/error503.php

In Drupal, you can use your tpl.php files to deal with returning status codes if you need to. For example, in page.tpl.php, you could do:

<?php
drupal_set_header('HTTP/1.1 503 Service Temporarily Unavailable');
drupal_set_header('Status: 503 Service Temporarily Unavailable');
drupal_set_header('Retry-After: 7200'); // in seconds
print "This page is temporarily unavailable";
?>

Or set up whatever logic makes sense... For example, if all blog pages need to be down and they have a url alias path like "blogs/xxx", you could do

<?php

if (arg(0) == 'blogs') {
// same code as above
return;
}
?>