This message was displayed because Moodle experienced a problem trying to read data from your Moodle database.
To get more information about the error you should enable Debugging in Settings > Site administration > Development > Debugging.
Setting this debugging mode will cause Moodle to display detailed information about the problem and a stack trace. You can then use this information and try searching moodle.org and the Moodle tracker to find any other reports of the problem with suggested solutions, or if necessary create a new issue for it in the Tracker. The Moodle developers will then use this information to reproduce and debug the problem.
After you have reported the problem reset the debugging level to the original state so that your users do not see the debugging messages.