I finally figured out why the index disappeared. after the server reboot , I found the previous indices in /var/lib/opensearch/nodes/nodes/0 !! ( 1 more nodes in the path depth )
I think Opensearch found that no indices could be found in original path in /var/lib/opensearch/nodes/0, it automatically recreated another set of index. Can I simply copy the index shard to original path and let Opensearch import it automatically ?