I'm trying to layer and mask. I use the brush tool, erase all the background, and withing seconds it reverts back to the original picture, like I've not done anything! It's not saving my work. Why?
A "full screen" screenshot showing the image the layers panel and options bar just after you have erased the background and immediately after it "reverts" may help us to spot what is going on.
And please make sure to include the tool bar. We also need to see what tool you're using.