You will want to go to the System Log and review the log entry mentioned there. You will want to test the URL which it was encountered on and see if the error still exists. If it doesn't, this likely was resolved by upgrading the core or third party application/plugin, an issue you resolved in your custom theme, etc... If it still exists, you will want to check the aforementioned items for upgrades, if they are available, you will want to upgrade. If that doesn't resolve your problem, you will want to disable third party applications/plugins and switch to an unmodified theme. If that still doesn't resolve the problem, please let us know.