What Can Cause Delayed Periods?

Hello doctor, I am 28 years old. It been two years of marriage. I always had regular periods but this month it is now 35th day from first day of last month but periods not started. I am having slight laziness and body pain from last 3/4 days. I have checked preg news and I am also not pregnant. Pls advise why this changes and any precautions needed.