I'll wager noone could tell you with so little information. A UserControl is just a control like any other, so if you've created it properly then there's no reason that you shouldn't be able to create an instance. Is it a WinForms control instead of a WebForms control? Is it in a different project that you haven't added a reference to? Your question is akin to "I have a car but I can't drive it. Why?". Could you answer that?
Ok JM . Its web Usercontrol. Sorry for my to much but even I it did not popup in my intellisense i got no error. But the problem is no control was created during i run my my web up.