First, rule out the obvious: check every character's bags + bank, the guild bank if you're in one, the auction house (unsold items return on expiry), and the mailbox. If it's still missing, open a Submit Ticket with type Other and include the character name, approximate item/gold amount, when you last saw it, and whether anyone else had access to the account. If the loss looks like account theft, secure the account first: change your password and enable 2FA before filing the ticket, so the account is clean by the time a GM reviews it.