mixer has disappeared

Can someone help please!! The mixer has totally disappeared…not just cured by F3 or devices dropdown, or even by system restore the computer. I just can’t think where it’s gone. I have also opened other, previous projects and it hasn’t showed up there either, when it always did! I was messing around with some shortcuts when it went, but nothing serious… and I’m wondering what on earth I’ve done. Can anyone advise? Cubase 6 artist by the way.