If your viewfinder is working I see no reason to not able to toggle between It and the LCD. Check the menu if preview is on or not..that is it will display photo right after photo is taken or shutter button is pressed
,it may be off to save power by previous user. Try toggle LV button on right side of viewfinder if it switches between LCD and viewfinder.Who knows the eye sensor circuit may not work but the LV button does.