Ouch. You can't really. Deleted in Jira really does mean deleted.
OnDemand has backups, but these are more for emergencies than looking for the odd mistaken delete. You could try a support call with them, but I don't know that they'll be able to help. (Also, consider severely restricting delete rights in the future. To maybe no-one until it's desperate and an admin temporarily enables it and removes it later)
...be more productive while being fun to use at the same time. For some, getting started can be a bit intimidating. This is especially true if Jira Service Desk is your first exposure to Atlassian...
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