I’ve run into this too many times, and sadly just now figured it out.
Typically the eraser cursor in Photoshop (or stamp tool, etc.) is a circle to show the size. I’ve seen from time to time though that it is just a cross hair; very frustrating as it doesn’t show the range you are covering.
I’ve played in the settings in the past, and I know there are settings there tied to this, but not my issue as I haven’t modified them and I’m having the issue.
But the shinning light came to me today! I had accidentally pressed the Caps Lock button. So when the Caps Lock is on, your eraser cursor becomes a cross hair, rather than the circle I’ve come to know and love.
I hope my ramblings help you out! I’m sure I’ll end up looking back at this my self.