You don't say how old you are.
If you are under the age of 40, you do need to make an appointment to see your doctor to check your hormone and iron levels and check you out for any polycystic ovaries or other womb problems.
Over the age of 40, it may be due to the menopause and not having a period for 3 years will mean that your menstrual cycle days are finished.
Still see your doctor though to make sure that your period have stopped because of the menopause.
Are you in menopause ? Time to be checked and have a Pap smear at your Gyn.