Is it possible to "push" a leaning concrete wall upright?
Absolutely, but hammering won't work very well with that much mass. You can simply flex a stout board, like a 2x10, downward, then release it. The straightening action will push the wall. Leapfrog a couple of them so that one is always holding position.

A couple caveats:


Concrete is insanely heavy. Expect it to fall the wrong way and be safe. 
Have a solid plan for securing it in place once you move it.
Never work alone.