Is everything configured correctly in /config/site.php? If so, you might also want to try logging in to your database using MySQL WorkBench or phpMyAdmin to make sure there aren't any server issues.

This can also occur after a failed installation, or occasionally after uploading new /concrete files but before running the upgrade script.

In these cases you can go to /index.php/tools/required/upgrade to run the script. However, this does not appear to be the problem on your server right now, so I would look into these other possible causes.