THis has happened to me as well. The issue here is that i have left the company that i was at and now all my stuff is gone including all the ArcGIS Pro stuff i had been testing under my user. now they have all my credits and i can no longer gain access.
You can change the ownership of multiple items and groups among other Bulk Actions many administrators are looking for with Admin Tools for ArcGIS Online is here https://marketplace.arcgis.com/listing.html?id=c34019b0623041608df4d06970a7a96a
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