You don't say if it is a modern muzzleloader. If it is a modern muzzleloader, take the breech plug out and push the bullet out along with the old powder through the breech. Clean it, inspect and determine if it is safe to fire.
If it is a primitive muzzleloader without a removable breech plug, a bullet extractor attached to the rod will be what you need to remove the bullet. Clean, inspect and determine if it is safe to fire.
Personally, I wouldn't fire it. You may be okay but the potential for corrosion over 10 years of sitting loaded would make me cautious.