I’d like to share this resource with you perhaps to aid in determining an answer without proper veterinary care / necropsy performed.
Normally in these situations it is necessary to get a necropsy performed to determine the cause of death so you know exactly what you can do so you can prevent it in your other birds.
Sometimes due to genetics, factors of egg handling, no vaccinations performed, etc that may be outside of your control, you just end up with a high mortality amongst your flock.
http://extension.msstate.edu/agriculture/livestock/poultry/diseases-poultry
I would also like to share the idea of telemedicine or telehealth. If you don’t have a vet nearby, you could seek one via phone consultation or hotline. There are websites where vets post actively who could potentially help you.
https://www.justanswer.com/bird-vet/
Chickens are flock animals, and the stress on your remaining chick being alone should be considered for its health. This stress will put this bird at risk.
Introducing other birds right away would not be ideal. I would quarantine this chick in a seethrough container (with air to breath of course) but so it can see other flock mates. You’ll need to introduce new birds so this chick isn’t alone but also so that you’re not putting them at risk, too.
Sad for your losses, and I hope this can get figured out for you.