Do you have a ball puller? It's a screw that attaches to a ramrod. If so, remove the barrel and take the nipple out. Soak the breech end in hot water for 15 minutes and use the ball puller to pull the ball. Make sure there isn't a second load (for safety), then clean the gun. You soak the breech end in the water to deactivate the powder.
If you don't have one, go to your local walmart (if they carry muzzleloading goodies) and get one.
Doing that or using a CO2 discharger are the safest methods. A CO2 Discharger uses CO2 to blow the load out the barrel. If you had one you would have used it already.
There is a third method, but you have to know exactly what you have. Did you load the gun yourself? Did you check to see if it was unloaded first?
If you did both of these, then you can possibly use the third method. Remove the nipple and trickle a few grains under it and replace. At a firing range, place a cap on the nipple and attept to fire it. If the gun does not fire fulley (expel the ball) then push it back down and place a little more powder under the nipple and attempt to fire again.
DO NOT ATTEPT THIS IF YOU DID NOT LOAD THE GUN YOURSELF OR VERIFY THAT THE GUN WAS UNLOADED FIRST! THIS IS A DANGEROUS METHOD! WHEN IN DOUBT PULL IT OUT! Sorry, I don't like this method and hesitate to mention it without adding the warning.