I am guessing the powder got contaminated. Be sure and pick the nipple real good. Push that nipple pick in as far as you can reach. Try another cap and see if it fires. If it does not;

Take the nipple off the rifle. Then take some powder in your fingers. Slowly drizzle the powder into the open nipple hole. Take a tooth pick and kind of pack that powder down there. Try to get in as much as you can in there but not up level. Remember you have to put the nipple back in there. Now replace the nipple in the rifle.

NEXT TAKE YOUR RAMROD AND MAKE SURE YOU SEAT THE BALL. SHOOTING A BALL THAT HAS MOVED OFF THE CHARGE CAN BE DANGEROUS AND POSSIBLY DAMAGE YOUR RIFE. MAKE NOT OF WHERE THE RAMROD ENDS OUT THE BARREL SO IF THE NEXT STEP WORKS YOU CAN MAKE SURE THE RIFLE IS EMPTY.

Now you can shoot the ball out in most cases.If the ball fails to come out, then repeat the steps sooner or later the ball will come out of the barrel. Make sure you point this is a safe direction because in your case the rifle is loaded, but actually if you dry ball the rifle, you pack enough powder in there, I have had them fly 25 yards hit a target and bounce back at you.

Another way is get to an air compresser and remove the nipple. Turn the compressor up to as high a PSI as it will go. Put an air pressure attachment and rig something so you can force air into the back end of the rifle. Sometime you can blow the whole works out..

I normally shoot them out since I live in the country.