Hello Swapna,
Do you mean all fields don't work?
Only fields with F4 value help might have this function. And also, some fields get disabled from application level.
Please read descriptions in SAP Note 1821509. It gives more explanation.
Please also check the personalization setting->personalize layout->if 'disable the suggestion of entries for input helps' is checked. If yes, pls uncheck it.
Best Regards, Corrine