Hey there. Please see a topic I’ve made documenting things similar to this.
However, I would note that in your particular case it sounds like a charge back occurred on the loot boxes you purchased.
I’m not too knowledgeable on charge backs but I think they are essentially refunds. Check your bank account for any refunded payments or see if you have loot boxes again. I believe when charge backs occur they can cause items to vanish and most definitely cause negative credits.
I know it’s not a fix but any information for you I would hope is useful.
Topic: https://us.forums.blizzard.com/en/overwatch/t/skins-missing-or-becoming-locked/1566