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 ?
Try holding your Shift key as the computer boots. This should force the GRUB menu (the place you selected your previous version) to show, and you can then choose the newer kernel and try booting into that. Unless there is any particular reason (kernel module needs patching or similar), it's probably better to just stick with whatever the newest kernel is for your install.