From your description, I suppose it is a private channel which has a standalone Sharepoint site, and it has been removed by the channel owner.
(By default the owner won't be able to leave the channel unless he assign another owner, or he may need to delete the channel)
Was the channel removed in the last 30 days?
If yes the Team owner or the channel owner can restore the channel, which would also restore the associated Sharepoint site and the files.
Please refer to this link: Private channel SharePoint sites
How to restore a deleted channel:
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