As Your Ultrasound
is normal,You need following investigations:
TSH, to rule out thyroid problems
FSH, to rule out premature menopause
Prolactin, to rule out a pituitary tumor which inhibits ovulation, and thus menstruation
Estrogen and progesterone
levels, to see if you're making the right hormones for cycle.
You are given a "progesterone" challenge test. If you're making estrogen, then if progesterone is given, then withdrawn should start a period.otherwise no estrogen is being made.
Treatment will depend on cause.