How can I turn back the App Store icon if I removed it?

It can't be deleted as it's a built-in app. If you can't find it on any of your homescreens or app folders, and you can't find it via the spotlight search screen (swipe your first home screen to the right), then is it hidden by Settings > General > Restrictions > Installing Apps being set 'off' ?

If not then have you tried a reset to see if you can find it after the iPad has restarted ? 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.