HI
Thanks for posting your query to HCM .
        There are quite a few causes for such Unconscious spells . I will tell about few common cause . 
1. 
Seizure Disorder :    A condition which causes convulsions ( fits )
2. Stroke               :    Inadequate Blood Supply  or Bleeding inside the Brain .
     Cause for Seizure is 
Electrolyte Imbalance ( Low / HIgh Blood Sodium, Calcium Magnesium levels , Low / HIgh Blood Sugars ), Infection spreading to Brain , Some Mass in the Brain , Epilepsy , Liver Failure , Kidney Failure . 
  Cause for Stroke are 
Uncontrolled Hypertension , 
Uncontrolled Diabetes , Elevated Cholesterol , Smoking , Alcoholism , Disorders in Blood Coagulation . 
    I advise you to get MRI Brain , Serum Sodium , Calcium , 
Lipid Profile , 2D-ECHO and revert back with reports .