Quick and easy way is to create a simple html file, and call it
app_offline.htm, and paste it to your website root directory. Note that the file has to be named as app_offline.htm
Once IIS finds this file, it stops further processing.
To make your website online, just delete this file or rename this file
<!-- Sample app_offline.htm file -->
<html>
<HEAD><title>Web Site Down for Maintenance</title>
</head>
<body>
<H1>Your custom message blah blah blah</H1>
<BR>
<!--
To prevent IE 404 error, I am adding few lines of commented text
<h2>IE Cannot handle files less than 512bytes in size. It works in Firefox and Chrome ...</h2>
<h2>IE Cannot handle files less than 512bytes in size. It works in Firefox and Chrome ...</h2>
<h2>IE Cannot handle files less than 512bytes in size. It works in Firefox and Chrome ...</h2>
<h2>IE Cannot handle files less than 512bytes in size. It works in Firefox and Chrome ...</h2>
<h2>IE Cannot handle files less than 512bytes in size. It works in Firefox and Chrome ...</h2>
<h2>IE Cannot handle files less than 512bytes in size. It works in Firefox and Chrome ...</h2>
<h2>IE Cannot handle files less than 512bytes in size. It works in Firefox and Chrome ...</h2>
<h2>IE Cannot handle files less than 512bytes in size. It works in Firefox and Chrome ...</h2>
<h2>IE Cannot handle files less than 512bytes in size. It works in Firefox and Chrome ...</h2>
<h2>IE Cannot handle files less than 512bytes in size. It works in Firefox and Chrome ...</h2>
<h2>IE Cannot handle files less than 512bytes in size. It works in Firefox and Chrome ...</h2>
<h2>IE Cannot handle files less than 512bytes in size. It works in Firefox and Chrome ...</h2>
<h2>IE Cannot handle files less than 512bytes in size. It works in Firefox and Chrome ...</h2>
-->
</body>
</html>
