The proper answer is "Send it to Nikon or an authorized repair facility". It will be expensive to fix, and may exceed the value of the camera (to you), but they will give you an estimate first and you can decide if it is worth fixing. You can try calling first and describing the issue and they may (or may not be willing to) give you a "no less than $x" cost based on what they observe, but as with any repair they may find additional issues.
You could try gently seeing if your finger will allow the mirror to release, tug on it a bit. This may do more damage, and it is doubtful it will solve the issue, but I did have a camera once that due to a blow the mirror jammed up, but worked perfectly after being released with a gentle nudge. Don't count on that working however, but if the choice is writing off the camera, it may be worth trying.