Eyedropper tool suddenly stopped working

The eyedropper tool has suddenly stopped working. I tried resetting preferences and restarting PS, I've tried resetting the tool, I've also just run the most recent update - none of these have worked. If I try to select a colour with the eyedropper it shows as white. In the colour mixing dialogue box the colour shows as 'ffffff'.

I'm new to PS, so any suggestions would be gratefully received. And of course I have an imminent deadline - software always plays up when you have a deadline!