Investigate elasticserach logs on the node. Try to start the elastic again. Also it might be a network issue (elastic port blocked between cluster nodes). You can also contact support via builtin product assistance chat to help you get this resolved.
When we get this error as Elastic search is down, first thing I would check is that is there any yraffic spike has happened from RUM side amd also check the storage in the cluster nodes as well. After that you can check the logs of Elasticsearch to get the precise reason why it is down.
When I faced this error in one of my projects, the reason that Elasticsearch storage was occupied amd the disk was slow.