That is a Sylvania / Zinsco panel. How was it taking that panel cover off!
They are crap.
Breakers can fail to trip. Breakers weld themselves to the bus bars. Breakers have a tendency to slide down the bus bars and at times will slide all the way off and just dangle in the daylight! I no longer take the covers off of this panel. I have boilerplate that tells everyone about this panel and the issues it has.