This is probably because the previous user has changed the default settings so that the turtle is not shown. Never mind, just create a new one, by clicking on the turtle button in the taskbar, and placing it into a graphics screen. You can see what name it has by clicking on it with the right mouse button. And to make sure that the turtle will listen to you, type:
to the command line and press ENTER (where name is the name of the turtle - for example t1 or t2).