Internet connection problems are most commonly caused by disconnected cables or by routers or modems that are not operating properly. First, try running the Network troubleshooter to see if it can help diagnose and solve your problem:
Open the Network troubleshooter by right-clicking the network icon in the notification area, and then clicking Troubleshoot problems.
If running the Network troubleshooter didn't solve the problem, then follow the steps described in Wired and wireless network connection problems in Windows.
If the steps in that topic didn't help, then follow these steps:
Make sure that all cables are connected (for example, make sure your modem is connected to a working phone jack or cable connection, either directly or through a router).
Restart your modem, and then restart your router. Remove the power cable from the modem and router. After all lights on the device have gone out, wait at least 10 seconds, and then plug the modem and router back in. Some modems have a battery backup that prevents the lights from going out. For this type of modem, press and quickly release the Reset button. If you don’t see a Reset button, remove the battery instead.
Check your router. Because of the new networking features in Windows Vista and Windows 7, some older network routers are not fully compatible with these versions of Windows and can cause problems. For a list of routers that are compatible with Windows Vista, go to the Windows Compatibility Center website.
If these steps don't solve the problem, look for a specific problem and solutions to try in the following sections.
Try these things:
Make sure that your modem is turned on.
Check to see that the Ethernet cable is plugged correctly into the Ethernet port on the modem and on your computer. Each end of an Ethernet cable looks like this:
Verify that you are not using a DSL filter between the phone jack and the modem.
Check the lights on your modem. These can sometimes indicate the location of the problem—whether it is the Ethernet connection, the power to the modem, or the DSL or cable connection.
Winsock corruption can cause connectivity problems. To fix this, open the Network troubleshooter:
For more information, go to How to determine and recover from Winsock2 corruption on the Microsoft website.
Ask your Internet service provider (ISP) to verify that its servers are functioning correctly and that you have an ISP user account and access to the ISP service.
If you have recently replaced your router or modem, ask your ISP if it uses MAC address filtering. If so, your ISP must add your new device to the list of devices that can access the network.
Make sure that you are dialing the correct number, including any required access numbers (such as 9), and that the number is not busy.
Make sure that the phone jack is working. To test this, plug in a working phone and see if there is a dial tone.
Make sure that the phone cable is plugged into the "line" jack on your modem, not the "telephone" jack.
Make sure that the phone cable is not defective by plugging a working phone into the "telephone" jack of your modem. If there is a dial tone, the phone cable is working.
If you have call waiting, try disabling it, and then try the connection again.
Your ISP might have disconnected you if you weren't interacting with the website for a while. Try connecting again.
If someone picked up the phone while you were online, you might have been automatically disconnected. Try connecting again.
Most dial-up modems work only with analog phone lines. Verify that you have analog phone lines installed, or, if you have digital phone lines installed, verify that your computer has a digital modem.
Make sure that your modem is working properly. For more information, check the information that came with your modem or go to the manufacturer's website.
Contact your telephone company to verify the quality of your line.
If your computer has two network connections, the networking software must choose which connection to use for network traffic. The networking software picks the connection with the best performance. If connection A has Internet connectivity but slow performance and connection B has no Internet connectivity but better local network performance, the networking software will route network traffic over connection B. This means that you won't be able to view websites, so the networking icon and the network diagram in Network and Sharing Center will report that you only have local connectivity (no Internet). This is correct, but it's not what you want. To force your computer to use connection A—the connection with Internet connectivity—disconnect connection B and try again.