I would sit in a chair, muzzle pointed in a safe direction, holding the rifle by the forend and grip. Press the top lever over with the right thumb and bump the hinge down against my knee, gently the first time, then with a little more force.
Sounds like it was closed on a 'thick' rim/long case body and forced closed, or with an extruded primer perhaps causing the binding in the firing pin hole.
If uncomfortable in doing this, you need to find a gun smith who will tackle it. I can tell you, aside from running a rod down the tubes to see if it is loaded or has fired cases in the chambers, she or he will do the same as I noted above, to start, I am sure.