There are numerous reasons a computer might not be able to connect to the Internet. Troubleshooting the connection to find what is preventing the connection from going online can be a lengthy process, but most of the steps are simple and user-friendly. Modern computers allow multiple ways to troubleshoot your Internet connection from home without having to contact the technical support line of your Internet service provider (ISP).
Powercycle all network devices. A good rule of thumb for any Internet connection problem is to reset (also called "airgapping" or "powercycling") all devices. This will almost certainly be the first step your ISP's technical support department will walk you through, so it is a good idea to try it on your own first to save the time.
Locate the broadband modem your ISP installed at your house as well as the power supply that connects it to an electrical outlet. Unplug the power supply either from the modem or from the electrical outlet. You should see all the lights go out on the front of the modem. If any lights are still illuminated on the modem, you did not unplug the right cord.
Leave the modem unplugged for about 30 seconds, then plug it back in. At this point it is necessary to powercycle whatever the modem is plugged into. If the modem is connected to a router, switch, hub or other network device, unplug the power supply from that device and replug it after 30 seconds. If you have the modem plugged into one computer or other device used to browse the Internet (like a gaming console), restart that device.
Once everything has fully rebooted, try the Internet connection again. If the connection is still not going through. Take a deeper look at what is affecting the connection.
Bypass all network devices. Think of your home network as a long metal chain. Each device--the modem, the router, the computer--is one link in the chain. If just one link is broken, even if all the others are working fine, it will cause the entire chain to fail.
For this reason, eliminate all devices that go between your broadband modem and your computer to diagnose where the connection is failing.
To do this, locate the ethernet or USB cable that comes out of your broadband modem and, if it is connected to anything other than a computer, disconnect it and plug it straight into one of the ethernet ports on your computer. Powercycle the modem in the manner described above and test your Internet connection again.
If your connection works at this point, contact the technical support line of the manufacturer or whatever network device your modem was previously plugged into and have them troubleshoot the device to see what is blocking the connection from going through.
If the computer still will not go online, run some tests on the computer to diagnose your Internet connection.
Determine the IP address that the modem is sending to the computer. For Windows machines, open the "Start" menu and click on "All Programs." Click on the "Accessories" folder and right-click on "Command Prompt" from the Accessories sub-menu. From the right-click menu, choose "Run as administrator." This will open a black and white DOS prompt window with a blinking cursor. Type the command "Ipconfig" and press the "Enter" key. The command will probably output lots of text. Look for the number listed out beside "IP address" or "IPv4 address" for a connection labeled "Ethernet Adapter Local Area Connection," "Tunnel Adapter Local Area Connection," or "1349 Connection." If the numbers listed start with 192, 169, or are all zeroes, this indicates that the computer is not receiving a valid IP address from the modem. Press the up arrow key and you will see "Ipconfig" on the command prompt again. After "Ipconfig," type a "/release" and press enter again. Press the up arrow key one more time and delete everything after the forward slash. Type "Renew" so that the entire command reads "Ipconfig/renew" and press the "Enter" key. This should output all the connection information again. If the output now lists an IP address that does not start with 192, 169 or all zeroes, that computer now has obtained a valid IP address from the modem, and you can try the connection again.
On Mac machines, you can determine the computer's IP address by opening the "Network" icon from the "System Preferences" icon. The top of the screen will read "Built-in Ethernet" and will list an IP address in the text to the right. Again, you can recognize a problem by looking for all zeroes or any IP starting with 192 or 169. To get a new IP from the modem, click the "Configure" button and click "Renew DHCP Lease." Check if the connection renews with the same 192, 169 or zeros IP or if it provides a valid IP after renewing.
A valid IP should allow you to go online. If not, there might be a deeper connection problem.
Contact your ISP's technical support line. The reason your ISP staffs a technical support line is to make sure your service is working. Contacting them can immediately tell you whether you might be part of an Internet outage or if there is some other signal issue preventing your modem from going online.