I need to display a Default Page something like "Site under Maintenance" whenever I bring down my Application Server ( WebSphere). Is there anyway I can do this in websphere considering the fact that the application server is down
If your application server is down, then you can't do it through websphere. You need to check your internet settings, many sites have apache running in the background, so if you do then you can load a page there.
This isn't really a WebSphere question but a general server admin setup question. It all depends on how your website is being hosting on the internet.