If your stopper is still stuck in the bottle, use a mixture of two teaspoons of alcohol, one teaspoon of glycerine and one teaspoon of salt. Shake this mixture and then pour it over the stopper to loosen the area where the next and the stopper meet.

If you are at your wits end you would have the stopper drilled carefully out so you can get to the liquid in the bottle but you will need to have it stored in another container after that.