Argh! That's frustrating...
If you are the owner/creator of a Group or Event, only you can delete it. So assuming you didn't delete it, one other possibility is that when creating the Group or Event, you may have missed the "Save and Close" button at the bottom right. Depending on how long your Group or Event info is, this button can be pushed toward the bottom of the screen and can be overlooked. Try creating your Group or Event again, making sure to "Save" before moving on.
We hope this helps!
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Oliver @ Selectivor