What Causes Delay In Periods?

Hi, I have been on the nuva ring for 2 months. My first month I had no complications and had my period on the scheduled date with no problem. This month I took my ring out on the scheduled day (monday) and today is sunday. I still have not started. I'm concerned because I'm suppose to put my new ring in tomorrow and I don't know whether I'm suppose to or not. I have taken several pregnancy test and all have came back negative. No signs of pregnancy either. Prior to the nuva ring I was on the depo provera for almost 4 years and I needed something new that would cause me to have a normal cycle. I dont know if that has anything to do with it. -Susan