Ubuntu 12.04 upgrade, on going to previous version of Linux, ubuntu doesn't boot

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http://askubuntu.com – I upgraded my ubuntu 11.10 to 12.04. It had kernel version 3.2.0-24 and I went to the "Previous Linux Version" where I selected kernel version 2.6.38-11 option in the operating system options. After that whenever I start the machine, it shows startup ubuntu image and gets stuck to that. Nothing happens after that. I can go to the user shell and use commands. I can't use the graphical interface. How can I change the kernel version back to the 3.2.0-24? Or is there any other way to solve this problem ? (HowTos)