I presume that you have done a pregnancy test and this is negative.
If there is no pregnancy, you should make an appointment to see your doctor.
It may be a hormonal imbalance, polycystic ovaries or other problems. The doctor can arrange for a blood test to be checked for a hormone imbalance, and can arrange for you to have an ultrasound scan.
Over the age of 40, missing periods could be from starting on the change, or some of the problems mentioned above. It would still be a good idea to make an appointment to see the doctor.
Make an appointment to see your doctor to investigate why you have not had a period for 6 months.