I woke up throughout the morning with severe dizziness, room was spinning very fast, no nausea. Never had this ever. What is the cause?

Viral labrynthitis. A viral infection of your inner ear. Typically very sudden and severe. Best treated acutely with steroids and valium. With steroids the symptoms resolve quickly.
Labrynthitis. Most likely cause is labrynthitis - a viral infection of the inner ear. If no other neurologic symptoms (headache, blurred vision, slurred speech) - unlikely to be anything serious. It could also be benign positional vertigo. Both of these are self limiting and will get better. Either way, it would be a good idea to see your primary care doctor. If there is any hearing loss... See your ENT asap.