There are three possible explanations that I can think of:
1. You don't say how old you are; but, menopause can occur even in young women. It is usually not a concern until your 50's; but, it can happen at anytime. You might have had your tubal and then gone into early menopause. Not likely; but, posssible.
2. While the vast majority of tubal ligations are successful; some are not. You could be pregnant.
3. If your periods are usually regular, there could be something organic that is causing this one to be late. There does not have to be anything seriously wrong for this to happen.
No matter what the cause, I would see my GP (doctor) first with possible follow-ups with an OB/GYN.