The most likely cause for delay in periods in you,in absence of pregnancy
,seems to be your being underweight
The resumption of normal periods is not a problem as it can be done with the help of medicines but the most important thing is to treat your weight and it's associated deficiencies like deficiency of iron,calcium,protein etc.
You must consult your doctor and take a comprehensive treatment for your delay in periods and your deficiencies and less weight.
It would be very good if you can take the services of a nutritionist along with a Gynecologist.