Try accessing whatsapp after each step below
- Make sure data access is enabled' To enable data access go to Settings=>Wireless & networks then choose "Mobile networks".
- Refresh your network' you can refresh you network by switching your phone to Airplane mode and then back to normal mode (shortcut to airplane = Press and hold the power button until a popup screen with options appears=> tap airplane mode. It the same procedure to deactivate airplane mode)
Let us know if you still need further assistance.