Once pregnancy is reasonably ruled out with a test taken at the right time, the most common reasons for a delayed period are things that disrupt ovulation: significant stress, sudden weight loss or gain, very intense training, a recent illness, jet lag or shift-work changes, and starting or stopping hormonal contraception. Conditions such as polycystic ovary syndrome and thyroid problems can also cause irregular or missed periods.
An occasional late or skipped period is usually not a concern on its own. Cycles naturally vary, and one off month rarely signals a problem.
If you have missed three or more periods in a row, have other symptoms such as unusual hair growth, nipple discharge, hot flushes, or significant fatigue, or if you are trying to conceive, speak with a clinician. A negative home test can occasionally be wrong if taken too early, so a repeat test or a blood test through a doctor may be advised.