A 21-year-old man had a sudden onset of vertigo, nausea and vomiting 36 hours ago. He is unable to lift his head off the pillow because the room starts to spin if he does so. He has no loss of hearing, no tinnitus, and his ENT examination is normal. His pulse rate is 56 bpm and his blood pressure is 110/70 mmHg. What is the most likely diagnosis?