problem with Visio 2000 page view

I've been using Visio 2000 for many years, now, out of the blue, I find that the "Page View" has disappeared.

By this, I mean that Visio always showed the edge of the page  (A4 in my case) which meant that I wouldn't accidentally drop off the edge of the paper. Now the edge of the paper isn't shown.

Is there a known cause/remedy for this?

best regards

Alastair