Some MS Windows PCs infected by viruses over the last few years have had their DNS (domain name service, that which translates human understandable words into internet understandable numbers and facilitates all your internet browsing, email and etc) settings changed to point to a set of DNS servers initially run by the virus writers. Those servers were taken over by the USA FBI and have continued to run.
However the FBI servers will be shut down and when that happens, all the
infected PCs will stop working. More reading and a simple test tool are here:
PlaNet strongly recommends all Windows Operating System users to check your
own PCs and if they are infected and if they are there is more information on the above website about a remedy for the computer's infection.