smashed rhubarb :-(

I thought to myself over the last few days how well the rhubarb was growing, only to drop a heavy plank on it and it's smashed it to bits...  :'-(

Pull away any damaged bits so it doesn't let rot in, and it'll re-sprout from the crown.

It depends why it died back!  If it got eaten by something, dried out completely, or got broken off it'll recover.  If it has some sort of disease or something ate the roots, it won't.  You could try carefully scrabbling down to the crown to see what's down there still.