When I add a mesh on the left side of the 3D panel I used to see options for each mesh like amount of faces and size etc. for when I make a new mesh. But now that dialog is gone! I have the T panel open but it shows only Object Tools, Rigit Body Tools, Light Field Tools, and Cloud Generator. I don’t remember seeing any of these before. I must have accidentally pressed a keyboard button that hid what I’m looking for.
Not sure what happened, but you can go back to Blender default layout with File -> Load Factory Settings
As an alternative you can press F6, just after having added an object., it will make a popup window with the mesh object settings inside
It’s possible to hide the operator panel portion of the tool shelf. In that case it shows a small + button on the bottom and by pressing that you can expand it again. But yeah, F6 should work.