Hello dear
I understand your concern
Absence if 
erectile dysfunction could be due to 
defect in nerve supply, blood supply to penis.
Following are the cause of erectile dysfunction: obesity, 
hypertension, problem in back nerve, excessive alcohol and smoking, hormonal imbalnce/low testosterone
I would suggest to 
consult urologist and undergo routine blood tests like complete and diffrential blood count, ESR, CRP, serum testosterone, serum cortisol. color doppler USG of scrotum/pelvis to diagnose cause.
If all investigation come normal then problem could be due to psychiatric issue like stress/anxiety regarding future performance, fatigue.
It should require psychiatric consultation and 
counselling.
Meanwhile avoid stress, take healthy diet, drink plenty of water, do regular exercise for 30-45 minutes in morning followed by yoga.
Tablet CO Q and gonadil F
Do moderate 
masturbation regularly
Hope this may help you
Contact further if follow up needed
Best regards
Dr. Sagar