Lost items
If your account was compromised, you have the right to 1 account restoration. If it wasn’t they won’t recover your item.
Items don’t just disappear. One of the following could have happened:
1) Account hacked, and someone decided to delete that one item
2) You accidentally sold it to the merchant
3) You put it somewhere else in your Bank or Inventory bags.
Side note – you’d need to qualify for an account rollback if you were hacked.
In GW2, Trading Post plays you!