If you’re facing Wi-Fi issues with your Mac, you can use Apple’s Wireless Diagnostic Tool to (hopefully) help you resolve the problem.
You can open Wi-Fi Diagnostics from the Wi-Fi menu in the OS X tool bar. Try to connect to your Wi-Fi network, then quit any other apps that are open. While pressing the Option key, click the Wi-Fi menu.
Choose Open Wireless Diagnostics from the Wi-Fi menu, and enter an administrator name and password when prompted.
This opens the Wireless Diagnostics utility. Read the introduction in the window that appears, then click Continue. Wireless Diagnostics then tests the Wi-Fi connection to your base station or router, and checks your Wi-Fi configuration settings.
If no problems are found, Wireless Diagnostics lets you know that your Wi-Fi connection appears to be working. You can click Continue to see the summary screen. If an issue is intermittent, you can select the option to monitor your Wi-Fi connection to diagnose your connection further.
Of course, if all else fails you can call Apple’s tech support or make a Genius Bar appointment if there’s an Apple retail store in your neck of the woods.