I'm sorry to hear about your pullet.
Do you have photos of the brooder setup, of the remaining pullet and her poop?
It can be hard to know why one suddenly dies. At that age, Coccidiosis as mentioned does come to mind.
What do you feed, including treats and do you have an Corid (Amprolium) on hand?
Your remaining pullet may be lonely, but she should be o.k. by herself for a little while. Integrating a lone bird into an existing flock of adult hens can often be a challenge. Do you have room in the coop/run where you can section off an area for her so she can be near the others and they become a little more familiar with her?