Have you tried making sure that all the cords are plugged in? If they are, reset the modem and router.
If it's still not working call your ISP and see if they can help you fix your connection.
If it's still not working, it may be that your MacBook has a problem. Try getting internet on another computer. If all your computers have no internet, it's your router, modem, or ISP. If your MB is the only thing not getting internet, it's a software or hardware problem.
Hope that makes sense, it's my poor attempt at a trouble shooting guide.