Parts of my drawing vanish

Hi,

I'm trying to draw a couple of things fore a college project, but have hit a major problem.

When I open a drawing, the preview window shows a complete drawing made up of several parts, but when I open it, parts of the drawing have vanished.

See the picture of the drill in preview,

But when I open it, this happens.

All the layers are visable and unlocked.

I thought this was a one off, but The same thing happened on a second drawing after I saved and re opened it. I would appreciate some help on this, as Its going to make my life very difficult in doing my assignments.

Thanks.