A post in the Battle.net forums from Blizzard acknowledges that they are investigating why some Diablo III players are missing some items on logging into the action/RPG sequel, saying in most cases logging out and logging back in (perhaps repeatedly) will straighten things out. The problem is trickier for hardcore players, as apparently some have logged in to find their characters are missing altogether. This prompts them to offer the possibility of rolling back these missing characters:

We’re currently investigating an issue that’s affecting some characters, causing their items, armor, and weapons to go missing upon login. Although this issue is in most situations temporary and can be corrected with repeated login attempts, there are reports that some Hardcore heroes were lost as a result of this. For accounts which were specifically impacted by this issue, our Support team is investigating the possibility of providing a special one-time exception to allow for a rollback of the account, and subsequent restoration of the fallen heroes.

To contact our support with the request for this specific rollback exception, we encourage you to create a ticket through the following support portal with ‘MISSING GEAR DEATH’ in the body of the ticket:

https://us.battle.net/support/en/help/ticket/d3/297

Please know that the decision to allow such an exception is not an easy one to make, and that this is not a guarantee that we will offer the rollback to all accounts that petition us with this request. Only accounts which we can verify were affected by this missing gear issue leading to the death of a Hardcore character will be considered for these rollbacks. With that said, we do want you to know that we are continuing to address this issue, and ask for all players to be aware that the issue is currently ongoing.