Check the hammer's half cock notch, if it's broken off, you either need it welded up to recut, or buy a new hammer from VTIgunparts. A new hammer is probably easies to do, and the cheapest fix.
Another possibility is the trigger's sear, which if broken, may no longer fit into the hammer's half-cock notch. Replace the broken part.
The cylinder may not line up due to the possibly broken sear, as the hand won't push the cylinder far enough around.
Checking the inside parts might show that more parts need to be replaced, and as there aren't many parts in there to replace to begin with, and if it's been badly abused, it might need them Call VTI about parts.