If something you’ve ordered hasn’t arrived, you should contact the seller to find out where it is.
It’s the seller’s legal responsibility to make sure the item is delivered to you. They should chase the delivery company and let you know what’s happened to your item.
If your item wasn’t delivered to the location you agreed (eg it was left with your neighbour without your consent), it’s the seller’s legal responsibility to sort out the issue.
If the item doesn’t turn up, you’re legally entitled to a replacement or refund.