i have been participating in the overwatch twitch drop event and got my basic lootbox, i was in the overwatch menu and saw my lootbox button shining bright, but when i pressed it to open the lootbox, it said that i have 0 lootboxes, i restarted the game because of the issue and my lootbox completely vanished without me opening it. 
I just had this as well. for me it opened automatically, check your lootbox history, you should have still received the items.
Same thing here, but I was able to open the epic lootbox. When I came back to open the common one, it just was not there anymore…