Barring something horrible like a tumor, cancer or ear infection. I think it sounds like Peripheral Vestibular Syndrome. It sometimes happens to older dogs and it strikes out of the blue. The symptoms can be mild to severe. The most common symptom is a tilted head and loss of balance and circling. Some dogs may even vomit or refuse to eat. They also may have a side-to-side or up and down eye motion. It's upsetting to watch but most dogs recover without any treatment. The recovery takes a few weeks but sometimes it can take a few months. Sometimes dogs are left with a permanent head tilt.