Cannot allocate because a previous copy of the primary shard existed but can no longer be found on the nodes in the cluster

[wfeng888]

I have a cluster with two nodes , version es5.4.3. About 26 shards unassigned after machine down. I get these message

GET /_cluster/allocation/explain 
{
  "index": "dcvs_nonmotorvehicle", 
   "shard": 3, 
   "primary": true
}

The results：
{
  "index": "dcvs_nonmotorvehicle",
  "shard": 3,
  "primary": true,
  "current_state": "unassigned",
  "unassigned_info": {
    "reason": "CLUSTER_RECOVERED",
    "at": "2020-04-10T03:40:41.127Z",
    "last_allocation_status": "no_valid_shard_copy"
  },
  "can_allocate": "no_valid_shard_copy",
  "allocate_explanation": "cannot allocate because a previous copy of the primary shard existed but can no longer be found on the nodes in the cluster",
  "node_allocation_decisions": [
    {
      "node_id": "TklXzLKySf-czdu8zZ5hyQ",
      "node_name": "MYSQL2",
      "transport_address": "10.45.156.202:9300",
      "node_attributes": {
        "cname": "202",
        "rack_id": "rack_two"
      },
      "node_decision": "no",
      "store": {
        "found": false
      }
    },
    {
      "node_id": "WDaA85bmQhKZxvgq4ve0Kw",
      "node_name": "MHA-MASTER",
      "transport_address": "10.45.156.210:9300",
      "node_attributes": {
        "cname": "210",
        "rack_id": "rack_three"
      },
      "node_decision": "no",
      "store": {
        "found": false
      }
    },
    {
      "node_id": "dAXXDGDyQaGcex6VcZP0eg",
      "node_name": "MYSQL1",
      "transport_address": "10.45.156.201:9300",
      "node_attributes": {
        "cname": "201",
        "rack_id": "rack_one"
      },
      "node_decision": "no",
      "store": {
        "found": false
      }
    }
  ]
} 

I want to know why did "cannot allocate because a previous copy of the primary shard existed but can no longer be found on the nodes in the cluster" happen ? Thanks a lot.