1) Edit elasticsearch service unit file using the following command

sudo systemctl edit elasticsearch.service 
This command will create a file

/etc/systemd/system/elasticsearch.service.d/override.conf
2) Now, add the following lines in the unit file.

[Service]
Restart=always
3) Save the file .

ctrl+x  > Y > Enter
4) Refresh the unit file using command

sudo systemctl daemon-reload
5) Can check the changes using command

sudo systemctl cat elasticsearch.service

Link: https://stackoverflow.com/questions/52624720/how-to-auto-restart-elasticsearch-search-once-crashed-on-linux-server