Some dogs do normally loose their teeth as they get older and reach the end of their lives.
So if your dog is getting close to it's lifespan then it could be a normal thing that your dog is losing it's teeth due to bacteria and aging.
If your dog is younger then it could be that the dog has gum disease or periodontal disease which is not reversible once it starts.
You can still feed your dog some soft food when it loses it's teeth or add some water or chicken broth to the dogs hard dog food to make it easier for the dog to eat and digest.
If your dog just suddenly starts losing teeth and your dog isn't really that old then you should take the dog to see a vet to find out exactly what is going on.