prompt box - exists beyond my monitor

Hi I'm new to mac

I have a second monitor attached most of the time. I love the way it automatically reconfigures to one when I unplug the second monitor.

but..

Although most of the windows & palettes sort themselves out onto the single laptop screen. I will be working away and then notice I've been prompted by the colour picker box for example. Yet the box is nowhere to be seen... probably off to the right where my second monitor once was.

Unfortunately I can't do anything until I answer the prompt.
not even close down my graphic program
or save my graphic files.. so of course I can't even reboot

any ideas
thanks