As everypne has mentioned, a vet is in order. Some dogs hide when they are in pain.

Abother possibility could be something that happened or is happening at your neighbors if they are close enough to hear.

When I lived in a four unit flat I could not imagine what made my elkhound/shepaherd cross suddenly become extremely protective of me and our cats. When the maintenance man, also a tenant, came in to fix the stove she followed his every move with her teeth bared... Several days later he was arrested for beating his wife so badly she was hospitalized... I'd been at work while he was abusing her so I never heard a thing. Too bad the dog could not testify....