I don't know the cause of your problem, but haven't you tried re-adding the applet to the panel? Right-click an open area of the panel, and select "Add Applet To Panel...". On my system (Kubuntu 7.10), the applet is "K Menu". Assuming the panel is not locked (it's in the context menu), right-click the newly created menu applet, click Move, and slide it over to the left.