Print out the tracking info that shows the item as delivered then go to your post office and ask to speak to a supervisor, manager, or the postmaster. Show them the tracking info and ask them what happened.
The carrier could have scanned it as delivered to keep from getting in trouble if he/she was running behind and plans to deliver it in the next day or so.
Of course, the other options suggested above such as misdelivery or someone taking the package are also possibilities.