Has the data already been deleted? If so, submit a request to Splunk Cloud Support. They may be able to recover the data, but be prepared for them to say "no".
If the data has not yet been deleted then how to re-index data depends on how it was onboarded in the first place, whether the original source is still available, and what portion (if not the whole) of the data must be re-indexed.
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If this reply helps you, Karma would be appreciated.
