I used to have two little dogs, one a Maltese who was a female and the other whoes breed is unknown who was a male. They were very close in all ways

one day when I came home from school, I found My poor Maltese in an Alley way near the house. It seemed that she escaped under the gate whcih was unusual from the 6 years we had her, she never escaped. She was attacked by other dogs that we thought were the grey hounds we sometimes see around the place. Dont get me wrong they are great dogs and i dont have proof against them.

Tiger was very upset about this, he would just want to sleep all day and not play, he didnt eat all his dinner and he would just hang around. He eventually got over it with alot of TLC from all of us. Just two years ago we got another puppy, name charlie who is a border collie, they get along brilliantly, tiger is up to his usual tricks about sneeking food, and he boses Charlie around.
just give them time, and if they are really lonely consider getting them a friend