Do you know what the incubation temperature was? 24 days is pretty late but if it was incubated at below 99 degrees they might be a little late and still be OK. Did other eggs hatch on time and this one is lagging behind or is this the only egg you have that is due to hatch? Did you count the day they started incubation as day 1 or day 0?
I've incubated a handful of times and my little ones always came out by day 22-23, mostly they hatch out day 20-21 for me, I have heard of chicks hatching as late as day 25 though if temperatures were low. Can you hear anything when you hold the egg up to your ear (peeping or a tapping/clicking type sound)? Have you candled and seen any movement or if the baby has internally pipped into the air cell? I've heard the float test doesn't always work very well so it would be good to find out if there are any other signs of life.