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If possible, try moving your eye a little closer to the viewfinder. The viewfinder has a proximity sensor. If you are not close enough to the viewfinder it will not turn on.
This also may happen if the FINDER/LCD Setting option selected in the camera menu is set to Manual. To resolve this issue, you can either change this setting to Auto or press the FINDER/LCD button on the camera body.
NOTE: If model-specific information is required to complete any of the operations mentioned in this solution, refer to the operating instructions supplied with the product. Manuals are posted on your model support page.