Have you reached your maximum allowed mapped fields?
This limitation only applies to mappings in Kibana. Once the 1000 field limit is reached your data will still be indexed in Elasticsearch. You won't be able to search, visualize or alert on those fields in Kibana until they are mapped. To explicably map fields please see our lesson about about Field Mapping in the Parsing course.
Important note: your Elasticsearch index limitation is 15,000 fields. It is not impacted by the Kibana mapping limitation, which has been implemented to ensure good performance in Kibana.