Maybe it's the way the planets are aligned, or perhaps you have a bit of foul karma coming home to roost; but, sometimes a Photoshop tool will refuse to work for no apparent reason. Could be the Eraser, could be the Airbrush, could be anthing.
Here's a little secret: when a tool refuses to work, it's usually because there's an active selection somewhere. That is, there are marching ants afoot in your document and you just don't see them. Deselect everything by pressing Command + D (PC: Control + D), then try the tool again.