Circle Heads not showing in 3D Model

[jessicadance2000]

Hi, I think I've accidently changed a setting or hit something I shouldn't have, but now the circle heads aren't showing in the 3D model and are appearing as solid lines in the sections. How have I managed to do this, and more importantly, how do I undo it? 

TIA

[RDAOU]

@jessicadance2000 

It is the view scale. You must have changed by accident to 1:1 the Level datum got too small to be visible

Change the view scale to 1:50 or 1:100