Does your dog have a head tilt too?Is he still eating and drinking normally?Any vomiting?
Does he have a history of ear problems?
Any possibility of trauma?Any possibility that he may have gotten into and ingested something he shouldn't have?
I'm wondering if your dog doesn't have a case of 'old dog disease'. The real name isXXXXX which is also referred to as peripheral vestibular syndrome (the current "preferred name"), geriatric vestibular syndrome and idiopathic vestibular syndrome.
This disorder is more common in older dogs and thus the name geriatric vestibular syndrome -- but it can occur in middle aged dogs, too, so the name was changed. Idiopathic just means "happens for no known cause" -- so it is a good name but not the preferred one. It does sum up the situation well, though. For some reason dogs can suddenly develop vestibular disease. The problem seems to be due to inflammation in the nerves connecting the inner ear to the cerebellum (which controls balance and spatial orientation). It usually lasts between a couple of days and three weeks.
A few dogs have residual signs beyond this time, such as a head tilt. This disease normally affects dogs that seem normal up until the signs appear. Then there is sudden loss of balance with many dogs unable to even stand up. Rhythmic eye motion known as nystagmus is usually present. Dogs may be nauseous from the "sea sickness" effect of vestibular disease. Most dogs will not eat or drink unless hand fed or given water by hand because they have a hard time with the fine motor movements necessary to eat or drink from a bowl. As long as they are nursed through this condition almost all dogs will recover. There is no known treatment. Some dogs do have relapses but most do not.
Usually, if this is vestibular syndrome, it will clear up in a few days to a week. If he doesn't improve by the middle of the week, you might want to have your vet check him...just to be on the safe side.