The base was improperly prepared. Most likely they did not make it deep enough and fill it with sufficient aggregate to handle the huge load of the fireplace. As a result, the entire base has sunk into the subsoil.

Typical guidelines for base depth: 4 to 6 inches for walkways. 8 to 12 inches for driveways.

I calculate the weight of your stone oven at being potentially over 100K pounds of weight. That is like a large truck. You need a civil engineer to calculate the base requirements for something that heavy, and he should come out and establish the reason for the current base failure so that he becomes aware of other variables that might influence his calculations.

I think the entire project probably needs to be redone, but maybe others know if a crane could safely lift such a heavy fireplace temporarily out of the way. I doubt it.

Google mud jacking please.

@Joseph Corlett, LLC Mudjacking typically is used to lift a concrete slab in small areas where the weight of a four-inch slab is about 50 pounds per square foot. That is also assuming that compacted subsoil is only a few feet below your slab. In this case, you have a huge stone oven that might weigh more than 100K pounds. That might be exerting as much as 1500 pounds per square foot, and the slab that supports it appears to be many feet above the compacted subsoil, right at the edge of a retaining wall. If you mudjack that, what prevents the old slab and the new mudjack material from just continuing to descend through inadequate layers of base material?

I would have a civil engineer evaluate this, not a contractor.

At 14,000 psi, I'm pretty sure that fireplace is going plumb.

this was all done by my father in law who was a mason; and has since passed. this was his home. i beleive the way it was built was using anchors. here are some pics i found of when it was done. the actual wall itself is still perfectly straight and from my neighbors yard below nothing seems wrong. we would hate for anything to happen and the fire was hand bult by him so its very sentimental to us

If it was a brick oven you could just name it the "Leaning Tower of Pizza" and move on with life. But the base appears to have been infilled, and that is the problem as layed out by westes. You can try for a "mud jacking" and see if anyone will take the project on, but I would predict no one will want the responsibility if the FP moves differentially and cracks, or the retaining wall blows out. But worth a try and call some. Note it will probably just keep gradually sinking over time but I don't know at what point it becomes dangerous (like how much fill there was, whether you are in CA, or there is lots of water in the ground).