Human ancestor reproduction after chromosome fusion
Strangely enough we actually do have an answer for that.

There are a several examples of individuals who are born with two chromosomes fused together in all their cells. 

We even have a few examples where these fused chromosome is in a diploid state, ie all instances of that chromosome are fused in the individual. Such individuals come from families in small isolated villages that are very inbred. 

Chromosome number does not prevent fertility but it does reduce it in heterozygous state as it is now harder to get correct chromosome distribution during meiosis and thus fewer functional gametes are made. However in terms of function, as all genes are present in a somatic cell in the correct number, function is usually normal and the individual is alive.

If the population is inbred, it is possible to get an egg and a sperm that carry the same fused chromosome to meet. In which case you get a diploid individual with fused chromosomes. Fertility is now restored. And in an inbred population, a homozygote individual for the fused chromosome can meet another similar individual, and have offspring who are fully fertile.

AS you can imagine, over time there is a selection pressure for the population to separate. The homozygous state (be it fused or unfused chromosome) is more fertile that the heterozygous state.

So to put it short, chromosome fusion does not kill a cell. All gene are still present. However in the heterozygous state, where both fused and unfused chromosomes are present, fertility is dramatically reduced BUT is none zero. Fertility is restored in a homozygous state when an individual is diploid for either fused or unfused chromosomes. In small isolated populations, inbreeding allows for the emergence of diploid individuals which can reproduce with each other

http://genetics.thetech.org/original_news/news124