The most common reason for this is that someone has configured a backup location in the Dynatrace UI and forgot to add the mount point to /etc/fstab. If the backup mount is missing elastic search will not start. You can see this in the elasticsearch logs because it will have an ERROR that says path.repo is unavailable.
You have two options to fix this.
Add the NFS mount back to the Filesystem
Disable backups in the UI and restart elasticsearch. If you cannot access the UI or this doesn't work you can manually remove the configuration from the config file. The default location for the config file is /opt/dynatrace-managed/elasticsearch/config/elasticsearch.yaml
You will remove the entire line for path.repo and restart elasticsearch.