Some apps depend upon background data (transferring data when not directly in use). Restricting background data may cause those apps to stop functioning unless a Wi-Fi connection is available.
From a Home screen, navigate: Apps icon > Settings.
From the Wireless & Networks section, tap Data usage.
Under the Mobile section, tap Details: Mobile data usage.
Tap an app (below the usage graph).
Tap Background data to turn on .
Click here for info on setting data limits and warnings, changing the usage cycle on your device and viewing data usage by app.