As many as 45,000 Americans may lose internet access on Monday due to an infection on their computers. A criminal group called Rove Digital released the DNSChanger trojan, and made millions of dollars scamming their victims. They were eventually shut down by the authorities, but those infected with the trojan would have lost the ability to connect with the net months ago if law enforcement had completely shut down the servers that made the scheme work. In the US, the FBI worked with a non-profit organization, Internet Systems Consortium, to provide access for those who were infected, but that arrangement expires on Monday, July 9th.
So it is imperative to check your computer and make certain that you are not one of those infected. The FBI has set up a website where you can check for the infection. It can be found here.
If you are outside the US, you may have another checking site available for your country. The list of those sites can also be found at the FBI website. Click this link to see the list.
You can read about the DNSChanger Trojan at The Guardian.