Booting Custom Kernel

Dear Sir, I am currently trying to boot a custom kernel from source. 2.6.61.32. and tried to replace the kernel of an Ubuntu 12.04 LTS with kernel 3.11.X.Y. That computer also has an another Ubuntu installation 12.04 with same kernel as that of the another Ubuntu installation. Now , when I boot the machine and try to boot with the 2.6 kernel, I can't do that and Grub is not even mentioning that particular version of the kernel. If any help about this issue is possible , please drop me a mail. Regards, Arindam Pal M.Tech (CS), 1st year, Indian Statistical Institute ph no: +91 7685950229 <http://www.ixiacom.com/> <http://www.linkedin.com/groups?gid=150991> <https://www.facebook.com/arindam1991>