It looks like no one has responded to your question yet. Here are a couple of things that might help others answer your question:
1. Did you upgrade your iPhone to an iPhone 8 or did this happen after a recent software update?
2. Are you using iCloud for your calendar? Or is your calendar syncing with another type of account like an email account?
3. What steps are you taking to add your calendar and are you getting any specific error messages during those steps?