Evaluation. I think it is appropriate to be evaluated, but there is usually no reason to be worried. However, sometimes the evaluation will show things such as ovarian failure which was unexpected. So 99% of the time there is a common hormonal and easily treatable reason, but the evaluation is important to make sure everything is ok.
Depends. Skipping an occasional period is nothing to be concerned about. Not having a period for 6-12 months may increase your risk of endometrial hyperplasia or cancer - cancer of the uterine lining. It is best to take medicine to start a period at least every 2-3 months so there is not too much build up.