What happens if humans are missing a chromosome 23?

You don't have one. You have twenty-two pairs of autosomes, i.e. forty-four autosomes. You have two copies each of chromosomes 1-22. Your twenty-third pair is the sex chromosomes. If you're a female you have two X-chromosomes, you are XX. If you're male you have an X-chromosome and a Y-chromosome, you are XY.

If an embryo lacked any sex chromosomes it'd would probably abort.

45,X0 Turner syndrome

http://en.wikipedia.org/wiki/Turner_syndrome

The foetus would spontaneously abort.