Finally I have unlocked my iPad.
After posting the thread here when I did not get any response then I searched on Google and came across this useful guide.
The task which I followed to unlock my iPad, first I pressed & hold the top button until see the power off slider. Then dragged the slider for turning of and waited for a while for it to shut down completely.
It was a painful experienced I never forget. But now I feel relax to solve the issue.