Do not fill the crack with uncured clay and put back into the oven to re-bake. This generally causes the crack to enlarge. The uncured clay does not fuse with the already cured clay.

You can use a product called Plumber’s Putty–a two part epoxy clay-like substance used to repair pipes. It is dark gray, but can be sanded and under painted to match the clay. It hardens in a few minutes without heating.
You can also make “sawdust powder" from Sculpey clays and mix it with epoxy glue. This makes a paste of the same color as the clay, which hardens without heat. It can also be sanded and painted.

It is difficult to add on to any cured clay and re-bake. The cured clay does not re-melt and mix with the uncured clay, so separation will occur. Pieces that break off or were not attached properly prior to baking will need to be glued on.