There are many causes of unexplained fall. One is that the proprioceptive receptors of foot are at fault and they are not able to take in the variation in the floor. This can happen if you have high blood sugar or hypothyroidism affecting the nerve endings. The other possibility is an internal ear problem. It an also be due to pinched cervical spinal nerve. Brain injuries, tumors, transient ischemic attacks and bleed in the brain have to be ruled out in your case. It can also be a vasovagal attack. Hypoglycemia, severe anemia and vasovagal attacks are common causes of such falls. It can also be MS but other causes need to be ruled out. It can also be a sleep related disorder with extreme sleep deprivation.
Please consult your doctor regarding this immediately. Hope this helps. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.