If you are programming in VB.NET, your project has a Root Namespace property.  If this is filled in, that is the namespace that is used for your user control.

You should still be able to reference the control's code-behind class, even if it doesn't have a namespace.  You can also definitely add a namespace to the class, though make sure you update the Inherits directive on the .ascx control to match your change.

Is the code-behind class declared as Public?  Is it in the same assembly as the code where you're trying to reference it?

Hope it helps,