Please have a look at this sticky post:
In particular this section:
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, when the arena rules were reset (or earlier, such as the special Hallow’s End event, since you have been away for a while) 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.
Edit: Also, please do not double-post.