I started with modelling a tree and suddenly I am unable to scale vertices in edit mode for the branches mesh to go thinner or thicker. When I hit S to scale nothing happens. I don't know what suddenly went wrong. Please help
To scale the radius for the skin modifier you press CtrlA, not simply S.
There are also two fields in the properties panel, one for the X and one for the Y radius, "scaling" the radius with CtrlA does the same as changing these values.