I can think of two times someone has asked me to fix their computer when it has suddenly lost the ability to connect to the Internet. Assuming there’s nothing wrong with the modem or the network card, the problem usually comes down to something messing with either the TCP/IP stack or the Winsock. Security software frequently does this, as does malware. A few years ago, I briefly worked for an ISP that provided a security suite based on F-Secure, and that program was notorious for breaking the Winsock.
Here’s the simple fix.
Open a command prompt (make it an administrative command prompt if the computer is running Vista, Windows 7, or later).
Actually, I lied. Try the simplest solution first. Type ipconfig /release and hit enter, then type ipconfig /renew and hit enter. Try the Internet now. If it works, congratulations. If not, move on to the next step.
Type the command netsh winsock reset to reset the Winsock.
Open a web browser and try to go somewhere.
Failing that, try resetting the whole TCP/IP stack.
Open a command prompt (again, make it an administrative command prompt if the computer is running Vista, Windows 7, or later).
Type the command netsh int reset all to reset TCP/IP.
Reboot, then try again.
Some caveats: If the user has a corporate VPN set up, there’s a good chance this will break that. There’s also a chance this will break the user’s antivirus/security suite.
Since one possible culprit for this is malware, it’s a good idea to scan the entire system after fixing it. After doing that, reinstall the security software (or uninstall it and replace it with Microsoft Security Essentials).
Fortunately, it’s fairly rare for people to be running corporate VPNs from their home PCs–home editions of Windows are very limited in what they can do when connected to a corporate network, anyway–but if you’re fixing someone else’s PC, be sure to mention it, so as to keep yourself out of trouble.
There are utilities that will reset the Winsock and TCP/IP stack for you, but they won’t do you much good if you can’t connect to the Internet. Plus, you can type the commands in less time than it takes to download them.