is the above code on a movieclip that is in its own timeline has a frame called startscreen? you could try this.gotoAndStop("startscreen");
are you getting any errors?
are you sure the code is being executed? try putting in a trace like:
trace("im working");
above the gotoAndStop to see if the _root is findin what you need.
without knowing how you have set up your movie and if you are loading other mvies into it its ahrd to tell what is going on.