Although in the most recent ISO versions of Pandora FMS it is automatically activated in the system startup, Pandora FMS has at its disposal the pandora_HA file so that, when it detects that the pandora FMS server is down, it tries to pick it up.
For it, you will have to use the following command:
/usr/bin/pandora_ha -d /etc/pandora/pandora_server.conf
With -d you try to have the command executed in the background and then you have to set the path of the server configuration file.