[ubuntu] Cannot watch youtube videos

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http://ubuntuforums.org – hi... I was installed ubuntu 11.04 5 month before, starting i was installed flash player and it was working properly,,but form last 2 weeks i cannot watch youtube videos or anyother videos. its shows only blank screen in video side... and I tried to uninstall and install it ,but problem exists..can anyone help me for this, thank you (Hardware)