Hi, Have you been diagnosed with pcos and are you on any medications?
It is not normal to go that long without a period, I have the same problem i'm 25 with pcos and i don't have periods without medication. I would talk to your doctor not having a period increases your risk of uterine cancer. Not to mention your chances of conceiving again. I hope that helps.
It could a number of things. Do you have any other symptoms other then no period? I would consult your doctor with this. Your took young to have problems like this and you dont want to wait too long with it.