I accidentally magnified my game screen so I can only see a few tiles. How can I get it back to see the whole board
It depends on what platform you are using. If you are using a phone, use two fingers and pinch them together to zoom out. Spreading two fingers apart zooms in. Two fingers dragged up and down control tilt.
On a pc with a mouse, the center scroll wheel controls the zoom.
You might also want to check your settings to be sure auto-zoom and center is unchecked.