First, try the simplest cause: a dead or failing battery. On older Nikons, a weak battery can leave the mirror up and the camera unresponsive. Replace it with a fully charged battery, ideally a genuine Nikon battery, then turn the camera on. If you see an “Err” message, press the shutter button; the mirror may return to its normal position.

If that does not help, one community-reported cause on Nikons of this era is a shutter/mirror drive mechanism that is sticking due to dirt or dried lubricant. That points to a mechanical issue rather than a settings problem.

Because this involves opening the camera and lubricating internal parts, the safest recommendation is service/repair if a fresh battery does not clear it. Avoid spraying cleaners or lubricants into the camera.