How tragic! Dog attacks are among the worst chickens can suffer. And to lose them so young has to be tearing at your heart.
Chickens need a flock. If this is a solitary chicken with no other chickens, while "depressed" isn't quite the accurate word, it will be unfulfilled. If you have adult chickens, that will suffice as a flock as the chick matures.
If these seven chicks were all the flock you had, then it would be best for your surviving chick for you to get a few new chicks. But the new chicks likely won't ever bond with the older chick as they will with chicks they are brooded with. But they will all be a flock and derive security and a sense of well being from being part of multiple chickens.