Cubase Mixer has gone

I am using Cubase 5 on a PC and the mixer page has disappeared. I’ve tried F3, opening it from the Devices menu, minimising the project page to see if it’s hiding, but it is not there. I have checked all recent Cubase projects and it is the same for all of them - no mixer, even after rebooting the whole system. Mixers 2 & 3 are available, so it’s not the end of the world, but very inconvenient - and worrying if it’s a sign of major breakdowns ahead.

I have read elsewhere of someone having exactly this problem, but the thread ended without a solution being presented - can anyone out there shed some light on what is happening here?

DAW: Cubase 5, OS: Win 7 pro x64, CPU: Intel Core i5-3470 @ 3.2 GHz, RAM: 8GB, Graphics: GeForce 210