I just fixed this on my D5300. There’s a ribbon cable attached to the left side of the viewfinder that’s come loose.
Open up and remove the back, then unscrew the logic board and detach all the cables. You’ll need to do this to get to the ribbon cable. Take a look at that ribbon cable; it’s probably at an angle. Pop that cable out and reseat it and your D5300 will live again.
Note that the ribbon cables are kind of a pain to manipulate. Tweezers or needle-nose clamps might work better than fingers.