I agree that we need to know if it's an inline or sidelock.
Also, is the 50 grains of Pyrodex loose powder or a pellet?
Do you know how to clean a nipple by using a pin, needle, thin wire from a small paper clip or a nipple pick?
Placing a few grains of any type of loose powder under the nipple usually always works.
Do you have any loose powder?
If not, a pellet could probably be crushed in a dish to make some loose powder.
It needs to be ground very fine.
If it's a sidelock, the cleanout screw can be removed and the loose powder can be placed under that instead of the nipple.
There's other steps that can be taken to try to ignite the powder.
Go slow before deciding to pull out the load unless it's an inline where you can simply remove the breech plug and push it out using the ramrod or a long wooden dowel.