Help!!!!! My dog killed 2 of my chickens and now I think one is hiding

It is instinct for a chicken to remain quiet & still after dark. You probably won't find it tonight. Hopefully if it survives without being found by a predator, it will be nearby and will be ready to rejoin its flock mates. They will probably all be traumatized, though. You will need to be SUPER careful and tie your dog up or put it in a bedroom and close the door when you go out to check on the chickens. It sounds like you're in for some really serious retraining regarding the dog. You should consider making a secure run so you can let the chickens out to forage and have "outside time" while keeping them safe from your dog, a neighbor's dog, a stray dog, cats, possums, etc. BTW, welcome to BYC. I'm sorry for your losses.

ETA: it's a good idea to train your chickens to come to a vocal call or the sound of treats. Lots of people train their chickens to come by shaking scratch grains or corn in a can and calling "here chick chick chick!" or "Treats!". Whatever word you choose, but chickens are guided by their stomachs -- if they think food is involved they'll hightail it to you from wherever they happen to be hiding, unless they're too hurt, too scared, or broody. Good luck.