What would happen if the chromosome number were not reduced before sexual reproduction?

you will have a baby giraffe.

level 26 down syndrome!

Than that means that there would be a birth defect or disalbility causing the baby to die or have a diesese/disability

if the chromosomes dont balance your child will have some sort of problem the most common on is down syndrome

You could have one of several different birth defects and/or die. If you're human.

If you're a plant, it doesn't make a difference.

the pregnancy would not be viable.  it is doubtful whether sperm and egg would fuse at all and if they did the body would spontaneously abort pre implantation.

You talked about unreduced chromosome number (ploidy level). not about individual chromosome (non-disdjunction; uneupoplyploidy). In plants, this phenomenon is NOT uncommon. Many plants can prodice unreduced  pollens and eggs through what is so called first division restitution (FDR) during meiosis I and second division restitution (SDR) during meiosis II. Both FDR and SDR produce diploid (2n) gametes. When unreduced pollen (2n) fertilize unreduced egg (2n) then a tetraploid zygote (4n) will be produced.  Diploid potato (Solanum phureja), for example,  produce high frequency of unreduced gametes. Potato breeding programs are usually done in diploid level to achieve the desired traits; and then using unreduced gametes the normal tetraploid potato (4n=48) can be produced again. In addition, some scientists believe that the formation of polyploid plants in nature was also through these mechanisms.