Check the monitor display settings.

• Models with a LIVE VIEW/OVF switch
  Confirm that the switch is not set to OVF.
  If the switch is set to OVF, set it to LIVE VIEW.

• Models with a FINDER/MONITOR (LCD) switching button
  Press the switching button and check if the display appears on the monitor.
  

  NOTE: If you turn the camera off with the display mode switched to Viewfinder, the camera will stay in this mode when you turn it back on.

• Models other than the above
  While looking into the viewfinder, check the FINDER/MONITOR setting on the menu.

If the issue is still unresolved, the product may need servicing.