It sounds like you may need to refresh yourself on the Kibana + Elasticsearch security docs. I can try to point you in the right direction.
All user accounts are stored in Elasticsearch. We require certain roles to be able to log in to Kibana: maybe your elastic user has been downgraded from a superuser?
There are built-in user accounts when you install the stack, and you are meant to configure the passwords at install time. In particular, the elastic user is a superuser, and depending on your version you may also have a kibana user or kibana_system user.
You are supposed to update kibana.yml to configure security, and this is how Kibana connects to Elasticsearch.
You should also check what roles your users have.