you have probably started a crack that hasn't quite completely broken through. the below process works quite well for this sort of thing.
1) support the sill with something below it. this will prevent further movement.
2) get yourself a glue syringe from a place like lee valley tools. inkjet refill syringes work fine as well. use the largest syringe needle that is in the kit.
3) get a drill bit to match the syringe needle diameter
4) drill holes every 1" or so along the length of the crack to a depth just short of the full depth of the board
5) fill syringe with woodglue and inject into the crack. you need to insert the needle into the hole to make a tight seal with the hole. the goal is to squeeze glue in and have it spread into the adjacent crack that the hole straddles. you may need to thin the glue with water a little to achieve injection. you will just have to experiment. ideally, the glue will ooze out of the crack on either side. do every hole.
6) let dry, sand, fill, sand again and then repaint