Hi Sir or Madam,
Dogs can lose their voices just like humans do. In other words, they get laryngitis, which is an inflammation of the voice box and vocal cords. It can be caused by barking too much, infection and other causes. If your dog had been coughing, or has a runny nose and other cold like symptoms, your dog may have an upper respiratory infection. If this is the case, your dog should be seen by your vet.
If your dog does not have any other symptoms, I'd try to keep him in an area where he doesn't normally bark much so his vocal cords can rest and recover. If your dog's loss of voice continues for more than a week, you will need to have him seen to ensure there is no physical problem such as a growth causing the problem.
This can be an early sign of myasthenia gravis as well. I hope this information is helpful to you.
He dosent seem to have any signs of coughing or runny nose. He's usually barks all the time, so it seems unusal. Could it be related to over barking?
What is myasthenia gravis? IS this a possible sign?
Sorry about the delay. Overuse can lead to a loss of voice.
Here is a site on myasthenia gravis.