The simplest troubleshooting for such issue is SMC + NVRAM reset.
Intel-based Macs: Resetting the System Management Controller (SMC)
About NVRAM and PRAM
You would be surprise that you don't have to tear apart the computer and the reset resolved the issue.
Also you may want to load up System Information to see if the airport card is being detected by the computer.(click on menu, hold down option and click on System Information/Report)
If it's detected and no significant fault/error found, you may want to boot to the recovery system(Command + R on startup) and see if wifi works there. And if it works okay on recovery system, your issue is most likely a software issue and not hardware fault.