I would bet your trigger can be fixed.... The percussion hammer type Knights it is pretty common for the trigger to stop working unless you clean them as you clean the rifle. It would be my suggestion that you remove the trigger from the rifle and spray a good cleaning fluid like Birchwood Casey 2-in-1 bore cleaner through it.
There are also 2 adjustments on the trigger that might need to be re-set.
I also should ask:
1. When you pull the hammer assembly back does it catch the sear and remain back or does it go completely closed when you release it?
2. Or does the trigger refuse to pull and release the hammer assembly?