Is it just on one image or on any open images? I know in the past I have had problems using the eraser tool... but it was only on one specific image (and not all of them). After many frustrating hours I finally figured out that I had a very tiny selection on the image (I couldn't actually see) and it was keeping me from erasing anything. I just did a Ctrl+D to deselect and then I was able to use my eraser again.
The only other option I can think of is to reboot. I can't think of anything else.
I hope you figure it out.