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1. Open Photoshop and select the Zoom Tool from the tool palette.
2. Look to the left end of the Options bar and you'll see an icon for the Zoom Tool. Right-click on that icon, and select "Reset Tool" from the context menu that appears.
The reason is that you have the (-) sign selected in the Options bar instead of the (+) sign; you can either deselect it or reset the tool as outlined above.