Why my wifi is getting disconnected again n again?

[Windows Central Question]

I am using lumia 830 mobile and also using whatsapp application in it..
But whenever i use the data using wireless and after the lock screen, i am not getting whatsapp messages. My whatsapp goes disconnect and my wireless won't work after lock screen.
There are no option to use wireless after the phone is locked in wireless settings.
How can i resolve it.??