How to solve wifi is not working in Ubuntu 13.10 and older versions and derivatives eg LinuxMint

This problem happens to due to either your systems hard switch off or soft blocked
If hard switch is off turn it on and check the wifi connection is now working if not, open the Terminal by pressing keys Alt+Ctrl+T and type or copy and paste the following command in the Terminal:

sudo rfkill list all

It list all your system supporting features eg. Bluetooth, wifi as following screenshot

In above screenshot bluetooth is soft blocked so it can turned on by doing following:
Now copy and paste following command in the terminal:

sudo rfkill unblock bluetooth

or all features can turned on by one command as follows:

sudo rfkill unblock all

Now check again if all soft blocked Bluetooth, wifi etc turn on or not by copy and paste following command in the terminal:

sudo rfkill list all

Now you can surely connect to your wifi network or bluetooth devices.