How to fix my Nikon d90? - 1

My Nikon d90 camera has an flashing Err message on the top screen. When I half-press the shutter button, r13 comes up, and the mirror in the body won't stay up. I can't do anything with it, and the main LCD screen is always off. How do I fix it?

Added (1). The SD card is in the unlocked position, and resetting it doesn't work.

Some possibilities could include an SD card that is switched to the locked possition.
It could be a lens making a defective electronic contact.
If you are using an older type AF-D lens with an aperture ring, you need to turn the ring to the highest f number to make it work with the DSLR.

Reset it.

The manual should tell you the recommended procedure,
but with most gadgets, removing power for a few seconds will do it.
[Sometimes the power must be disconnected for a few minutes.]

I looked in my D90 manual. It basically says "ERR" means a trip to the camera hospital.