If it's a managed window (you've created it with System.Windows.Forms, and it has inherited from System.Windows.Forms.Control) you can get a reference to the Control object with
Control.FromHandle(myIntPtr);
Then you just get the parent of the control until you get the form.
If not, you can't get a Control object, what you can do though is to is to create a NativeWindow, and assign the IntPtr handle to the object with the AssignHandle. That will at least give you some access to the WndProc, and similar, but that's it.
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