Dual boot missing ubuntu 12.04 LTS ,now only Windows is directly starting?

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http://askubuntu.com – Previously i have window7 and ubuntu with dual boot. But now window 7 is directly starting there is no option for ubuntu (no boot manager) but i checked via live cd ubuntu partiton ext4 is there and all files also but it directly not starting and windows also not determine that partition or ubuntu boot loader entry ? (HowTos)