First things first...
Reboot.
If that doesn't work, turn off your pc again, reset your cable/dsl modem (if you have one), then reset your router (if you have one) and then boot once more.
If that doesn't work then open up a command prompt with "cmd"
ping yahoo.com to see if you get a reply...
If you do get a reply your dns is okay
If not, it's your ISP's fault. Call and complain and/or change your DNS to OpenDNS' servers.
208.67.222.222
208.67.220.220
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