Do you mean that you had an arena run in progress when the arena changed a couple of weeks ago, and that you did not see the card pack which is awarded at the end of an arena run? If so, look at this section of the Common Issues sticky post:
As stated, during the change of arena “season”, the pack should have been added to your collection right away. Perhaps you opened the packs without realizing where it came from, since there have been a few promotions recently awarding a pack or two.
If you go to the Arena rewards screen next and open the chests, all the rewards within (card pack, dust, gold) were already added to your account June 3rd, not when you opened them.