I do not know how with the sprint you have already deleted. Probably, it will never be returned. But in order to avoid such a risk in the future, you can make a backup of your project once a day.
settings / System / Back up for cloud. (Download the archive to your HDD).
In addition to themes, Jira maintains a history of activity. It is available on the system dashboard. This will not help you to restore the sprint, but after spending a day you will be able to get all the activity from the removed sprint. I assume my last note, but do not state it as a solution, because I (myself) have not used this method yet.
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