Colours suddenly disappeared in Photoshop

Hello

I was working this evening when suddenly I jogged my arm and my mouse leapt across the screen and all the colour disappeared from the document I was working on. There is colour still in the Swatches, but none in my document. I have checked the obvious, like did I accidentall switch to grayscale, but that isn't the solution, I am still in RGB. There's colour still in the thumbnails in the Layers palette, so I don't think I've done anything serious. It's just some part of Photoshop that I'm unfamiliar with, whose button I've pushed.

Thanks for your help

Best wishes

Matt