when i am clicking on my Ipad "app store" the message is coming that "cannot connect to itunes store"?

Can you access the internet in other apps e.g. Safari ? If you can then try logging out of your account on your iPad by tapping on your id in Settings > iTunes & App Stores and then log back in and see if it then works.

If it still doesn't work then try a soft-reset : press and hold both the sleep and home buttons for about 10 to 15 seconds (ignore the red slider), after which the Apple logo should appear - you won't lose any content, it's the iPad equivalent of a reboot.