Do you have a photo of this?
When crabs do this they are considered soft shelled. I think they do this on a yearly or so basis, depending on how fast they grow. They are very delicate at this stage and you probably do not want to disturbe it too much now. You might want to think about takeing out the other fish incase they peck at him/her/it.

the pincher and crusher should grow back maybe something in your tank found this to be a tasty treat and stole them from your helpless crawdad.Hopefully your crawdad will be able to bet some food without his front end.It may take months for him to become his old self again if you have some aggressive fish you may want to isolate him but make sure he can still filter some things out of the gravel anyway just noticed this post is old so whats the update on this guy.