Chick not eating/drinking and looks weak

We received these chicks from the post office today and they all seemed fine (assumed 2 days old?). My parents decided to give them some expired baby parrot feed as substitute while we went out to buy food for them; bought some small seeds and grain for parakeets as temporary food while the chick feeds comes in the mail. All the chicks seemed like they really like the seeds, except one who was sleeping in the corner and refused to move. It refused to eat or drink, or even stand up while I'm holding it. It looks extremely weak or tired, and I have tried feeding it sugar water. But after that it has vomited clear mucus twice and now looks even weaker than earlier, with open mouth breathing. I don't think the cause of that could be overheating since I have temporarily separated it from others (and the heat lamp) to keep a close eye on. There are no signs of pasting. What can I do for her right now?