she forgot to take Mercilon 21, and we had unprotected sex

Hi,
Welcome to ehealthforum,
If this was not her first birth control pill pack use, and if she has been taking her pills at scheduled time daily (and has not missed taking any active pill and not taken the pills more than 2 hours late), she should have the protection from the birth control pills. Wait and watch for her periods/withdrawal bleeding. Look out for development of any new symptoms if pregnancy is suspected. She should visit her gynecologist for further evaluation and assistance if she does not get her periods within 10 days of expected period dates, or gets abnormal bleeding. She should be regular with birth control use as instructed (taking the pills at scheduled time daily). Avoid having unprotected sexual contact if you are not ready for pregnancy (use additional protection if having intercourse, as none of the birth control measures give 100% protection).
Take care.