Welcome. I understand, it is nerve wracking in the beginning because every hen is an individual and lays at her own time and frequency. Even after my first flock, I worry when it doesn't go the way it was "supposed to", and I think something is wrong. Often there is nothing wrong. Give it some time.
Hello and welcome to BYC! My very first chickens were ISA Browns, and I've had over 100 since then. I absolutely adore their personalities; they are so friendly, smart, curious and funny. It is normal for ISA Brown pullets/hens to lay big, beautiful, golden-brown eggs every single day. Which means that the fact one of your pullets suddenly stopped laying is not normal. Unfortunately their little bodies pay a very high price for such a high lay rate. It would be unusual for one of your pullets to already be having reproductive problems, but it is a possibility. Continue to observe her behaviors, and you can also do bi-weekly physical exams. As much as my ISAs are constantly underfoot, they don't like to be picked up, but you can do a wellness check when they are on the roost at night. If your non-laying pullet shows any signs of illness, don't hesitate to create a thread in the Emergencies forum and ask for advice. The medical experts on that forum are extremely knowledgeable and always happy to help.