OR, pull the nipple off the bolster, shake out all the powder you can get out of it. If it got wet, it might be caked up in there, so a toothpick might help to loosen it up. Dont use a sparking metal, as it "could" set off the powder... (that said, I use a normal safety pin or bent paper clip, which will spark, etc, etc.)
Dribble a light load, 20gr or so...of powder back into the bolster/chamber area. Make sure the threads are clear of powder, and replace the nipple lightly.
Reseat the ball/ramrod piece onto the powder. This is very important.... RESEAT the projectile onto the powder. You dont want air between the powder and ball.
Cap, and fire into a safe location where you can recover the parts.
Some muzzleloaders have a small screw below the nipple that can be removed for dribbling in powder and cleaning. That would be a better way than trying to screw the nipple back in with powder in the threads...
This method has worked for me several times..
RipT