This error happens when your hosting company (Bluehost, Hostmonster, GoDaddy, etc) disabled your site due to security or payment reasons.
It can also happen if the site went down or is within a maintenance Window. If you get this warning, try to run a real time scan here: http://sitecheck.sucuri.net
to see if the problem remains.
You can also contact your hosting company for more details (since it could be related to something they are doing).