It's a Bridgeport mill. The lever is used to lock the spindle in place.
It may just be the roll pin that secures the brake lock shaft that failed or fell out, as it appears to have completely disconnected from the brake mechanism. Or it may be a catastrophic failure of the shaft or brake collar. Who knows, I'm just speculating based on schematics available online.
Still ***** that someone just left it that way. I'd be worried that running it may cause more damage from broken pieces flailing about inside. It needs to be locked out and removed from service.