Late at night with foggy brain, I thought I would scroll through an IQ grid by swiping my touchscreen.

A was the parent of B and B was the parent of C.  Somehow by my swiping, I made C the parent of B!  But somehow B was still the parent of C.  And A was no longer the parent of B.  So neither B nor C appeared under A anymore.  I used Live Search to find B again, and found both B and C there.  In the Search Grid, with context parents on, it now looked like this:

C

  B

    C

      B and so on.

What a mess !!  How to restore the original order?  I was rusty.  You can't edit the ItemParent field in the Properties pane.  Finally I found the solution in a forum post by Pierre.  With B selected, under Parents in the Properties pane, I right-clicked C and clicked "Delete Parent Link".  Hooray, C is no longer the parent of B.  Then I clicked Item > Marked Items > Mark Items to make B a Marked Item.  Then I selected A and clicked Item > Marked Items > Add as Sub-Items.  Now A was again the parent of B, and B was the parent of C.  

I was very glad to find this solution - until then everything I tried just made the situation worse.