I think my puppy is depressed! Because all of her brothers and sisters have been given away! What are the signs of it and what do I do about it?

How long has it been? They usually get over this pretty quickly. Distract her with a new toy!

Is it possible that you are feeling sorry for her, and passing those feelings on to your pup? Dogs will pick up on your feelings. Get your pup interested in other activities. Play with her a lot, exercise her, and establish your role as pack leader. She will be fine in no time. :)