Hi Carminator,
When was the arena run started? Because of the big changes to arean with the year of the Dragon, I presume that all runs which were not completed when the year started (April 9th) were forced to terminate, as is often the case in such a situation. If that is the case, please have a look at this Common Issues post:
Especially this part:
Did not get Arena rewards?
When an Arena run is forced to retire due to special limited time events (like Wildfest, or the Blizzcon special arena cards), the rewards are put directly into your collection when the run is retired. The opening of the chest is a purely visual thing.
What this means that in that situation, as soon as the sever stopped your run on April 9th, it would have given you the dust, gold and cards it owed you. Not a pack, but rather 5 cards, already opened, already put in your collection.
If this does not describe your situation (if you started your arena after April 9th), it might be a similar situation, or it could be a bug. I recommend opening a support ticket (use the Support link above) and a representative can have a look at your account. On this forum, we are mostly regular players like you helping each other out, so we do not have access to your account.