Take the top/side cover off (4 screws on side) and you'll be able to see the release mechanism. There will be a small white plastic piece which is blocking the release mechanism.
Push that piece down with a small screw driver while you push the open button.
This happened to me when I tried to play a sticking tape.
I think it thinks it's still in play mode and prevents you from opening the cover.
steve gallagher
March 2006