Why would we need to bring our site "down", and make it unaccessible to the world? The most common reason is that we need to alter something in IIS or on the file system, and a visitor being on the site may keep you from performing the task at hand.
Be aware though, that there are many ways to take a site offline. But if you think it would be a better idea for DNN to do it for you, read on. Keep in mind though, that this method brings the site down for everyone. Even you. If you need to bring it down for everyone except you, then I suggest implementing IIS IP address restrictions.
This is going to be a very, very, very, VERY complicated process, so be sure to pay extra special attention... (not really)
The first step is to create a static HTML file with information regarding the outage. A company logo and a simple "server undergoing scheduled maintence" message should do.
Here is an example that you can use:
Now, just save this on the root of your site with the filename of "app_offline.htm". Once you do, any requests reaching the DNN directory will be responded to with your HTML file. Just be sure you edit the META tags to tell the search engines to not index, and to return in a day.