This is a broader, sysadmin/.service monitoring question. If Splunk is not running, then by definition it cannot create an alert, can it?
You don't specify what platform you are running on (Linux or Windows). If Linux, I would suggest you probably want to use an external service monitor (such as Nagios or Zabbix), which will continuously monitor according to criteria you set (in this case a TCP service listening on port 8089), and alert to different levels (e.g. warning when the service goes down, critical after 5 or more minutes of unavailability).