can't access certain websites from ubuntu 12.04

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http://askubuntu.com – I am using ubuntu 12.04.It was absolutely fine 2 days ago.But now all my excitement is turned into frustration.I can't open facebook,yahoo,hotmail and many other websites although many other websites just work fine and even i dont have any problem in downloading.I looked up various solution provided on this forum but neither workout for me.Plz help me anyone or i have to uninstall this OS (HowTos)