What could cause a dog to become paralyzed in front legs?

Unfortunately, nothing good. I would first suspect trauma such as being hit by a car damaging the nerves in the shoulders. Other options include nerve inflammation, cancer, infection of the brain or spinal cord such as rabies or listeria, a blood clot in the spinal cord, etc.

You should take your dog to a veterinarian as soon as possible - the sooner a diagnosis is made and treatment started, the better the prognosis for return to function.