Eraser Stopped Working

[Connolly Music Creations]

I've been erasing the background on an image (saving frequently) but then out of nowhere the eraser stopped working.  I can still change the brushes, sizes, opacity, etc., but it doesn't erase.  On the toolbox under Eraser the Mode says Normal and I can't change that, but I'm so new I don't know if that's typical or not.  Since I shut down and came back to working on it, is there a history I can go back to and pick up from there?  Thanks!