Sometimes happens that right clicking the network manager tray icon, the option to enable the wi-fi is not selectable; this arises because of the hardware switch or because of a soft block of the wi-fi device caused by some system bug ( a  faulty driver or other kernel module can lead to connectivity loss) . The solution is easy:

1. Connect to the Internet using a cable
2. if not installed, install rfkill using

   sudo apt-get install rfkill

3. using the newly installed tool type

   rfkill list

   This command will output a list of all the wireless devices,their codes and their statuses, telling you if some of them has been blocked. If the block is a soft one (means caused by software) just type

   rfkill unblock all

   to unblock all the devices, otherwise just use:

   rfkill unblock device_code (the one from the previous list)

   to unblock one device.

4. if the block is a hard one, try to find out where the switch has been placed and use it.

Enjoy Ubuntu!!

Cheers