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 mode. In Moodle 1.x you do this by setting the Debug messages option under Site Administration > Server > Debugging to the DEVELOPER level. In Moodle 2 this option is under 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 to log an issue on the Moodle Tracker. The Moodle developers will then use this information to reproduce and debug the problem.