D.net run maintenance every night around 4 AM or so which means that the site is down for about 10 min and on occasion a half an hour to an hour.
as for you seeing a web page, and then clicking on a link and getting the maintainence screen.. that means your browser settings are not correct..
You browser is currently set to Automatically load a page.. which means that if a site is currently offline, your browser will load a page from your Temporary Internet Files and will display it and make you think the site is still online..
To fix this issue, at the top of Internet Explorer, click on Tools and select Internet Options
Look at Browsing History and click Settings.
Move the dot from Automatically to Every Time I visit the Webpage
You can also lower the ammount of space that Windows reserved on your HDD for temporary Internet Files...
It is also recommended that at least once a week, go in Internet Options and click Delete to remove form data, temp Internet files, and history.. I would leave Cookies and Passwords alone unless you like putting them all back in on your favorite sites. you will get a pop up window where you can delete specific things or everything.. I usually leave cookies and passwords alone otherwise I have to re-log into each site I visit regularly.
Hope that this helps.