Some users are reporting an issue where they keep on seeing this message after logging in. The reality is that usually there is no maintenaince happening but that this is cached in the browser.
Some browsers are very persistent and somehow won't show the actual content of the page. Solving this is quite easy though.
How to get rid of this message?
Luckily this is easy! Simply clear the cache/cookies of your browser, restart the browser and login again. If you're not sure on how to do this please follow this link which will explain you how to do it properly: http://www.wikihow.com/Clear-Your-Browser's-Cache