Migration and Upgrades in Progress...

If you see this message for more than a few hours, try the following:
What URL are using to access your website?
Yoursite.com or WWW.Yousite.com or Linda.Yoursite.com
Basically, there is a change your browser or router has cached the old location
BUT!!! You could simply type a different version, as above...
If you use Linda.Yoursite.com and see this screen....
..Simply type Yoursite.com without Linda. and then login.
If you normally type Yoursite.com and see this screen...
..Simply type WWW.Yoursite.com and then login.
You see, it's as simple as asking your browser to pull the website differently.
This should allow access for a few hours or a day, while your browser or router slowly update themselves.
Doing this, you could avoid Clearing Cache and History, which would be the correct solution
(I just like to avoid clearing my history and sites I login to)

But, if you want to clear, then do the following:
1.) Close all tabs but one, and go to Google.
..This is important, to NOT be on the site you are clearing.
..If you have your site open and do the next steps, you will not clear that site
a.) Try Control F5 in your browser
b.) Clear cache and HistoryOpen
c.) Open a Different Browser and type the site in again
Most folks will see the new site within moments, but some service providers
lag in their updates of new IPs, causing some users to have a delay in the change.
Cell/Mobile based service providers tend to lag the most.
Some MAC computers cache IPs of Browser history.