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)
If you spend enough time as a Jira admin - whether you are managing a single, mid-sized instance, a large enterprise one or juggling multiple instances at once - you will eventually find yourself in ...
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