A limping puppy is very concerning, especially if it happens all of a sudden without any obvious reason.
A puppy that suddenly starts limping could indicate mild muscle sprains, whereas gradual onset limps indicate serious diseases like hip dysplasia, but keep in mind that asymmetrical growth in puppies is also possible and often causes lameness.
A dog that shows no pain but limps could indicate that your canine is either a pro at hiding the pain or he is not seriously injured. If he refuses to put any weight on it then you should take him to the vet.
Examine the leg, if it is either swollen, hot, dislocated, or cracked at another angle, then you will have to bring your dog to the emergency room to avoid further complications such as infections or internal bleeding.