In case you have been low on disk space on your Elasticsearch instance, there is high probability that your indices are marked read only now. In order to fix this one, first either delete/archive indices or increase your disk space. After that restart Elasticsearch and Kibana and navigate to Management – Dev Tools and execute the following:

PUT /*/_settings
{
  "index.blocks.read_only_allow_delete": null
}

This should bring indices back to be writable once again.