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.