Most likely the wireless adapter is disabled. Go into System Preferences > Network. On the left select the Wi-Fi adapter. Click the gear icon at the bottom select Make Service Active and click Apply.
If this doesn't help the try resetting the NVRAM (PRAM). Turn off your computer, turn it on and immediately hold down Command+Option+P+R key combination. Hold it for a minute until you computer gives of a boot up sound a couple of times. The release the keys and let your mac boot back up.
Lastly if that doesn't resolve the issue it's likely a hardware problem with the Wi-Fi card. Bring your laptop to an apple store for them to take a look.