Marans eggs can be very thick. Or it's possible it hasn't developed a proper hook on it's beak. I would suggest very slowly and very gently getting the shell off but leaving the membrane intact if you can until you find the beak and try to keep it all moist with warm water. Stop there and see if it can make any progress. You could also break the egg around the circumference to help it unzip.
We've done this before but the key is to go very slow and steady. Don't panic, they have enough nutrients to live up to 36 hours inside the egg so it's not going to starve to death. See if you can get the beak out into the air and then take it very slowly from there. Don't be tempted to drag it out in one go. See if it can do it itself although it's probably shrink wrapped by now. Keep the membrane moist.
The biggest risk is that you will kill it by going to fast, not too slow. But you are probably going to have to help it all the way.
Also, if you do end up helping it a lot, it may kick it's legs a lot for a good few hours once out. It's the reflex for kicking out of the egg. Don't worry about this, it's not having spasms or anything. Leave it in the warm to dry out and fluff up.