WiFi is Disabled
Make sure that WiFi has been enabled in your system settings. To do this, go to your system settings and select "Wireless". Make sure you are connected to a wireless network.
If you are having difficulties connecting to a wireless network, try downloading the latest wireless drivers from lenovo.
If you've updated from stock Windows 8 to the newer 8.1, the Yoga 2 driver for 8.1 wireless needs to be installed.
Faulty Network Adapter
If the problem persists, then the network adapter may be damaged. To replace this, carefully remove the back plate of the computer to access the internal components. Make sure to remove the battery from the device to prevent any sort of electrical shock. Carefully remove the power cable from the wireless card and prop it out of the socket. Replace the device and connect the power cable.