If I save the document, the menus come back. In other words, Select and Mask is not working.
Is ‘Select & Mask’ itself not working, or are all menus greyed out when you work in Select & Mask? I am typing this on my iPad so I can’t check it, but I believe that greyed out menus are normal when you are in the Select & Mask dialog. That dialog has its own tools, sliders and buttons, and so the other Photoshop menus are not supposed to work at that time.
OK, start by giving more information. Which version of Photoshop, which OS. Don't say 'latest version', give a number. Then describe what 'not working' means. What does (not) happen when you try to use it? Like I said, most menus are supposed to de dimmed when the Select & Mask window is present, so what you describe sounds completely normal.
Like I said two times already: the menu behavior is perfectly normal. However, for some reason it seems that the Select & Mask interface is not showing. I noticed that your document is a smart object, so maybe try this again with a normal image just to see if that has anything to do with it (it shouldn't).
Try resetting the Photoshop preferences to see if that helps. https://helpx.adobe.com/photoshop/using/preferences.html