The vertical gridlines have suddenly disappeared from all my w/s.
 

The vertical gridlines have suddenly disappeared from all of my worksheets.
Horozontal gridlines are still present. I've tried
Tools/Options/View/Gridlines, but it is checked already. Even old saved
worksheets are now missing vertical gridlines. They will print out OK, but
are not visible when I'm viewing the worksheet on screen. Any ideas? I've
already re-booted my computer.....