I have a '67 Ford 4500 TLB. I was digging today when I heard a pop come from the boom. Looks like there is something wrong with the crowd cylinder system. The stick no longer moves - more accurately, the rod no longer moves. When I worked the lever, I heard what sounds like oil moving through the system under the deck. I don't see any new leaks or oil gushing from anyplace. Think something inside the cylinder broke? The pins don't look broken, and the cylinder isn't out of place.
If you're quite certain the loud pop you heard came from the boom components (like the crowd cylinder), only way to find out is take it apart. You could have broke whatever attaches the cylinder piston to the rod, although when this typically happens a cylinder will extend but won't retract. Keep us updated.
This is easy, lift the boom up so the stick is hanging free, and carefully push/pull on the bucket to see if the rod will go in and out of the cylinder with no resistance. If the bolt/nut holding the piston to the rod snapped, there'd be a nice hole in the middle of the piston to relieve the pressure that would make it extend by displacing the rod.
The rod is extended - the stick is hanging limp in the air. I tried to move the stick enough to see if I could move the rod in like Delmer said, but the whole thing is too heavy. I'll put it on the ground today and edge forward to see if it moves.