Congrats on the chicks! As long as the yolk sac has been absorbed, this little one should be ok. Make sure not to transfer chicks over to the brooder until they are fully dry, usually 12-24 hours after hatch.
Like @Debbie292d said above, no need to stress yet, 21 days isn't written in stone, they can hatch before or after this date. After they externally pip, it can take 24 hours to hatch. So hang tight and keep us posted!