If Rich's answer does not work, you may try hitting the "reset" button, which isn't actually a button, but should be a small hollow hole in the body of the camera. I tried to research online where this might be on your model, but could not find it. I'm assuming it has one, as most Nikon cameras do (I've owned the D40, D50, D60, D70, and the D300).
Use a bent paperclip to depress this reset button to reset the camera back to factory default settings. If this does not work, I'm afraid the camera will need a trip to Nikon for repair.
Did you happen to drop the camera at any time? This could have caused part of the body to break or dislodge and block the mirror from coming back down. I've actually had this happen to me before.
Good luck :)
