Can anybody tell me why my brush doesn't work anymore?

I am using the new Lightroom 2.4.1. (I wish it had a distinctive name, but so far I can't find one. It's the cloud based version.) Anyway, my brush was working fine, and suddenly it doesn't work at all. I've watched a million tutorials, I've tried many things on different pictures, I've gone over and over the settings. I can change the size of the brush, it shows the area being brushed (turns it a soft red), and then when I use the sliders absolutely nothing happens. I've restarted everything. I have Feather at 50, Flow at 80, Denisty at 100. (Although it doesn't matter what I have any of these set to.) This is terribly frustrating! HELP! THANK YOU!