Hard to say. I have had chicks hatch at 14 days and some that took over 21 but 24 days is a long time. Are you absolutely sure you've got the dates right? Is there any chance this one could have been exposed to sub-brooding temperatures which caused it to stop developing and go into a dormant mode as it awaited the temperature rising again? This can cause it to be developmentally younger than its chronological age.
I wouldn't have cracked the egg personally, though I do understand your reasons for doing so. I would have waited until there was at least a day's worth of the chick working to hatch after it had cracked the egg itself. Cracking the egg won't provoke a chick to hatch if it's not ready.
How do you know it's breathing? I'm guessing you can see its belly moving? If its beak is flat against the membrane but not trying to break it, I would guess (and I may well be wrong) that it would be best to break the membrane for it... Given how old you believe it to be. If you're sure it's as old as you think. If you're not sure, maybe candle it and see if you can see any un-absorbed yolk. If it looks like it's ready then maybe it is, but if it's taking no steps to hatch there is a chance that for whatever reason the chick is a dud. Recently a panda had a baby who looked normal but had no brain. Chemicals can have the most interesting side effects. Silkies, from what I've heard, often have duds, especially the closer you get to SOP. Just cracking the shell, just perforating the membrane, or even just hatching it won't guarantee it will quicken.
If you find more info from more knowledgeable folks would help. (Sorry I'm not that helpful here, just trying to offer what little I know on helping chicks hatch who otherwise wouldn't --- I have done that before). Best wishes and I hope it hatches. If it does hatch and does live though I would mark it in the journal as being a likely non breeder due to the risk it may pass this on. I'd probably test it but it could produce more of the same.
