I've had a similar situation with a cap and ball revolver. The cap went off but didn't ignite the powder (black powder substitute). The ball moved enough to lock the cylinder and prevent disassembling the revolver. This is a worse situation since there is no breech to open to limit the pressure. After waiting for a hangfire, I have used a cleaning rod held on the sides with a glove or towel to push the ball back into the cylinder. If anyone knows a better solution, I would like to hear it. I don't believe putting another cap on the nipple is a good approach, since shooting with uncompressed powder can result in too high pressures.
Paul