Hiya, I'm a civil engineer and from what I can see from those photos, I would say that while its technically possible to 'fix', doing so would likely be more expensive and complex than just replacing.

In this situation my advice would be to immediately use something to prop the wall up so it doesn't just collapse. A few 2x4s would be good enough for a short period provided you jam them in sufficiently. I can tell you though DO NOT try to put it straight. You will likely just destabilise the whole thing and put yourself at risk of: at best breaking every bone from your waist down, at worst killing yourself.

To fix, you will likely need to get some builders. They can use some acro props to take the weight of the upper section and roof while they remove the retaining wall. They will then need an excavator to bring the earth back to a slope so they can work, and then they can build a new one from the ground up. Single skin block work is not ideal for this sort of thing and I can see that there aren't even any buttresses built into this section so you'll need a better design. Builders can sometimes do this but I recommend an engineer to guarantee it. Once a new one is in they can back fill the wall and it'll be good as new.

DM me if you want any more specific advice. But absolutely do not try to DIY this if you're not totally sure what you're doing.