What could cause your dog to wake up with a limp even though he was okay last night before bedtime?

There are two major differentials for sudden onset limping - injury or disease. I would suspect an injury in a younger animal, such as twisting a joint watching the raccoons in the yard overnight. In an older animal, I would suspect the onset of Arthritis; the joints tend to be stiffer and more painful in the morning but usually become looser and more flexible with motion.

It is important to know that "sudden onset limping" as reported by the owner is usually not a true sudden onset limping. Often the dog has been showing minor signs of limping for some time, but because the owner sees the dog every day he doesn't recognize the very subtle day-to-day changes.

If this is an older dog, or this doesn't get better in a couple of days in a younger dog, you should take your dog to your vet for an examination.