If your Web browser, email program, or any of a hundred other Internet-connected apps on your Mac starts complaining about not having a connection, you may have to do a bit of sleuthing to figure out the cause. After all, a disruption anywhere along the chain between your Mac and a distant server could cause an outage, and it's not always obvious where to look.
I suggest trying each of the following steps, in order, until you're able to connect again.
1. Try another site or app
To make sure the problem isn't restricted to just one website, try visiting another--preferably one that's highly reliable, such as Google.com.
Similarly, to make sure the problem isn't just your current app (such as your email program or Web browser), try connecting to the Internet with another app. If only one site seems to be having problems, try visiting Down For Everyone Or Just Me and entering the problematic site's URL. The service will tell you whether computers elsewhere on the Internet can successfully connect to the site.
2. Use Network Diagnostics
Certain types of network problems may cause your browser to display a Network Diagnostics button. This is OS X's way of offering to help debug your connection problem, and I suggest accepting that help. (If you don't see a button, you can launch Network Diagnostics manually. To do this, choose Apple menu > System Preferences and click Network. Click Assist me, and then click Diagnostics.)
The Network Diagnostics utility will guide you through a series of questions and tests, ranging from checking your ethernet or Wi-Fi connection to network configuration and DNS servers. Sometimes the utility can repair problems itself; when it can't, it usually provides more detailed information about the nature of the problem and offers suggestions for solving it.
3. Nudge Wi-Fi back to life
If your Mac connects to the Internet via Wi-Fi, check the Wi-Fi menu (at the top right of your screen) to make sure you're connected to the network you intend to be. Macs have been known to hop onto less-desirable networks at inconvenient times.
If the menu's icon has an exclamation point, indicating that it can't successfully connect to any network, try choosing Turn Wi-Fi Off from the menu, waiting about 30 seconds, and then choosing Turn Wi-Fi On. If that still doesn't work, restart your Mac. Sometimes that's the only way to clear wacky errors that prevent Wi-Fi from connecting.
4. Try another device
If you have access to another computer or mobile device that uses the same Internet connection, check to see if you can connect to a website on that device. If not, you can at least rule out your Mac as the source of the problem and look elsewhere for a solution. But if the other device can connect and your Mac can't, even after a restart, skip ahead to the step "Check your DNS settings."
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