Symptom:
My scale bar and screen grid have disappeared, despite having the option to display them on in the Settings.
Resolution:
It is likely you have unintentionally switched into Perspective projection. To get your scale bar and grid back, press your 'O' key on your keyboard to switch back to Orthographic projection, or you can access this option from the Look drop-down in the top toolbar.
Root Cause:
The shortcut keys ‘O’ and ‘P’ allow you to switch easily between the different projection modes, however, it is very easy to inadvertently hit the ‘P’ key, resulting in a missing scale bar and screen grid. Objects displayed in Perspective projection are not to scale, and thus the scale bar and grid are hidden.
If this does not help, you may need to update your video card.
How to configure AMD switchable graphics for Leapfrog
Setting up NVIDIA switchable graphics correctly for Leapfrog