It sounds like what you are seeing is the Locking Bolt. Captured in the top of the Locking Bolt is the Firing Pin Striker.
My first action, if it were mine would be to start with a VERY thorough cleaning. It could be gummed parts not engaging or moving freely (a crudded and stuck striker would stop the hammer and not fire). At this time, also inspect for wear and breaks.
This could be a one part issue of a broken cross bolt safety that is blocking the hammer all the time, or it may be multiple issues, as in safety and either broken or stuck striker.
Even though I am known to say that cleaning guns is highly overrated, this is a time to slowly and methodically do a good cleaning and inspection, part by part.