If all the bar line have disappeared add a new instrument (any will do). That should have bar lines - click on one of the barlines where it meets the top or bottom of the stave. You should see a little blue box, drag the box up, or down across the other staves. The bar lines should reappear. You can then delete the instrument that you added at the start of this process.
I suggest adding an instrument that will appear at the top, or bottom of your score.