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Have you changed the eraser to background eraser or magic eraser. Or are you sure you have eraser tool selected and not the heal brush  or eraser mode set to block instead of brush opacity of the eraser. Or maybe it is another layer interacting with that layer. I assume your layer mode is set to normal. if all else fails close photoshop and reopen it as soon as it starts to load press control + alt+ shift and it should give you an option to delete your preference file and that will reset all your tools back to default.