Internet works during instal Ubuntu 15.4, but 'wireless device not available' on use

I have a Dell Inspiron 15 and I have installed Ubuntu 15.4. During the install process, the system coects to the Inteet and installs packages (except one). But on logging in, it says 'you are not coected to the Inteet' and the network manager says, 'wireless device not available', but this message is greyed.
I have run iwconfig and the wireless device, Qualcom Atheros, is shown.
Need guidance, please.