Fatal error class 'mage_helper_data' not found magento
(1) Kindly go to System->Tools->Compilation
Check your Compiler Status is eneble or not. If enable switch it as disable.
(2) Kindly go to Your module Helper file and check Class spelling for example :
Mage_Mycompany_Helper_data should be Mage_Mycompany_Helper_Data
