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    Everything was working fine, and after saving a simple Category change I got this error message:

    Fatal error: Class 'Mage_Hellothemes_Helper_Data' not found in /home/wildpopp/public_html/magento/app/Mage.php on line 520

    What is up with that?! The store is still visible:

    but the Magento dashboard is DEAD. What can I do???
    Please help.