Unfortunately there are a LOT of ways you can make something disappear in Maya.
Let's see... you've already tried Show All. Also try Show Selected, just in case... also make sure the layer that the object is in is set to 'Visible'... also open the Attribute Editor for that object and make sure that the Visible checkbox is checked (under Object Display)... also switch to another camera to see if it's visible from there; maybe you turned on 'Isolate Select' in your primary camera.
Don't forget that every object has two nodes, a Transform node and a Shape node. You can turn off the visibility on either of those.
Finally, if the object is a polygonal mesh, make sure you don't have any weird settings in Custom Polygon Display that would make it go invisible.
Good luck!
-Steve