Could you check if you have added your widget in the panel or not?
See if your widget(Label) is added or not. If not added, please verify that:
1. you are adding widget in the panel not other place on your mashup(try to add widget in )
2.Verfify that your session is not time out; if so relogin again and try to add Label widget again.
3. You could also add Label or other widget in other place other than the same panel, but prerequisite is that you added another panel(or if you directly add a Label widget in other place than the same panel, it will inform you that a new panel will be added in order to place the Label widget).
Hope it helps,