As Zekewolf said, the most common reason a case gets stuck is due to lack of lubrication. Two things come to mind in your case. You might not have lubed that particular case as well as you thought, or more probably since you stated/implied it was very tight in the rifle chamber, the lube might have been wiped off the case in the lower end of the die leaving the case dry in the die itself.
Methods to get it out?
Buy or borrow a stuck case removal tool. RCBS sells a good one.
I would have to look at your die to know if the following method would work. If you can remove the seating stem leaving the case mouth open, you can screw the die in upside down and push the case out with a strong bolt through the case mouth and touching the head. Use the leverage power of the ram to press the shell out. A little penetrating oil (e.g., PB Blaster, Liquid Wrench) down the side of the case should help.
You don't say what type of press you have. You might have to remove the threaded sleave in the press if it has one in order to get the die in upside-down and then replace it by screwing it into the press while simultaneously screwing the die into it.