In SharePoint once a site collection is deleted it moves to the Recycle bin. It then remains in the Recycle bin for 30 days before it is removed from the Recycle Bin.
If one wants to recover the site collection, it has to be done within 30 days. It can be recovered through the SharePoint Online Admin Center.
Once the site collection has been removed, there is a possibility of recovering it in the next 14 days. One would have to submit a support request and Microsoft would then recover it for you.