Recently my mirror on my Nikon D3100 became stuck. Nikon quoted a silly price to repair so I looked on net and followed advice and gently unstuck mirror with a tiny screwdriver. Now my light meter is not working. Hugely overexposes photos and keeps saying subject is too dark. I can get correct exposure if I override either shutter speed or f stops. I haven't pressed compensation buttons — all is set to zero and never had this problem before. I've owned this camera over 4 years. Any ideas?
Check to see if the focus screen/view screen has become dislodged a little, or if something is stuck to the focus screen. The exposure meter is located up past the view screen in the same area as the pentamirror. If an obstruction is blocking or filtering light from reaching the meter you will get the behavior your camera is exhibiting. Also, if the focus screen was removed when you got the mirror unstuck and accidentally put back in upside down (the side that should be facing the light box/mirror is now facing the pentamirror/viewfinder) it will affect the amount of light that passes through the view screen.