I just ran into this issue and found that @magic-code's response was correct, but let me clarify the steps a bit:
1- In App Store Connect, click the "Apps" text link in the very top navigation bar.
2- Find your offending app in the list (I had just one so it was easy)
3- Is there a red notification bubble in the top-right corner of the icon? If so, click directly on that red bubble.
4- You should be taken to a Resolution Center page with a message from the review team on why it was rejected.
In my case, it was rejected for not providing an email/password account they can use to access all corners of my app. I didn't realize the review team was going to review the app at this step or I would have provided it, but hey, I'm glad they to know they'll be checking it now. They even provided a screenshot of my app at the Login screen, which I thought was cool.