Thats weird, if you had torn it off then it would be grayed out in the list of panels you can open. Also the "Warning: Panel size cannot accommodate all requested Heads Up Display elements" is just to tell you that the panel is resized to be too narrow to show your heads up display items, it has nothing to do with that problem. Attach the UV Editor back to main Maya window/close any instance of it and try again. If not, maybe your prefs are screwed.