Suppose, I want to execute unix commands. For that I have to go to ksh, but if I don't have execute permission to ksh itself then is there any way to change the permission of ksh? chmod command does not work for this because, I don't have permission to ksh itself... 
Let me know, if you have any idea.
If you don't have permission to execute ksh, then your administrator might have ksh locked down for some reason.
First, what shell are you running?
# echo $SHELL
Second, do you know the path to the ksh executable? What is the existing mode? On my system:
# whereis ksh
ksh: /usr/bin/ksh /usr/share/man/man1/ksh.1.gz
# ls -l /usr/bin/ksh
-rwxr-xr-x 2 bin bin 315440 Dec 5 2006 /usr/bin/ksh
As you can see, the ksh file is executable by everyone. I can't think of a reason why your systems administrator would lock down ksh to a more restrictive mode. Give me an answer to the 2 questions I asked and we'll see where we can go from there.
ksh is not locked. I am working on a project where I am using a different shell rksh. If I want to go ksh then I will type a command & enter into ksh. but the problem is that the ksh is built in a binary CEC which I used to replace frequently. while doing it once, I moved the existing binary & placed a new binary then exited from ksh. But after that, from my rksh I can't execute anything as I don't have execute permission to the CEC binary. My rksh commands internally calls that CEC binary, so If I dont have x permission to CEC binary then there's no way except reinstallation.
Is there another shell you can log into? Are there other users with accounts on the box who could help you?