That looks like powdery mildew to me, or some other fungal disease. I'm not sure what can be done at this point - it looks pretty advanced. I've heard that foliar spraying with milk can slow the progress, but again, your plant looks to be covered with mildew already. If you have other squash plants nearby, I would maybe pull that one (on a still day and carefully, avoiding spreading spores as much as possible and washing up well afterwards to avoid spreading it) and bag it.
Mildew spores are ubiquitous in the environment; what causes a plant to succumb like this is often some kind of stress (weirdly, not having enough water is often a culprit, even though this is fungal). Another helpful thing I've heard about is aerated
compost tea applied foliar-ly - but I have yet to put together a good system for that.