If you have administrative access to your ArcGIS Online organization, then you can check your activity log.
If you haven't created an activity report before, check out the documentation for ArcGIS Online, Create and access reports. You will want to create an "Activity" type report for the date range during which you think the group was deleted.
The report will be returned as a CSV file, which you can download, and then look for a row that has:
- Name of the group in the data column (stored as JSON, e.g., {"name":"Name of Group"}).
- "delete" in the action column.
The actor column will indicate who preformed the deletion.
Another helpful resource for exploring the activity log is this blog post, Exploring the Organization Activity Log CSV.
