Thanks for the additional information Bill, I really appreciate it.
I'm not sure that this is a stroke. From your description, I actually think Zach is having an issue called geriatric vestibular syndrome.
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, although many vets will send out medications to help with the nausea associated with the head tilt and the nystagmus (shimmering eyes).
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. However, since she is an older girl, it may be best to have your vet take a peek in the morning and get her on some anti-nausea medication and/or fluids to correct any imbalances he may have from not eating or feeling well.