Fatal error class 'mage_helper_data' not found magento

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