Of course you can demand a replacement.
Whether you get one or not depends on a host of factors.
Your dispute in the first place is with the person who sent the item.
If a trader they should either replace the item or refund your full costs (including shipping).
If it was a private shipment from a friend they should contact Hermes and report it lost. Whether and what Hermes refunds depends entirely on the shipping conditions agreed at the time of dispatch.
Yes. THe item was not received, therefore the seller is responsible for providing a replacement or a refund. If this costs them money, they need to take it up with hermes (and also never use Hermes again – they are an AWFUL company).
Having had trouble with hermes before, I can say you have to go directly to the seller. Hermes do not have a contact or support line for recepients, and won't deal with you directly.
It depends what coutry you're in. In the UK the law is clear that the issue is between the seller and the courier.
You can claim a replacement from the seller.
Demand it from whom? You didn't say. You didn't even say what the item was, or how you know it was stolen.
If you didn't buy insurance, you are probably SOL.