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.
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