Immersion is not valid until the seven clean days have been completed.

If the couple have not yet had relations (because they realized the error in time or for any other reason), she just needs to count her 7th clean day and immerse again.

If they have already had relations, she needs to wait four days (the day of relations is the first of those four days) while continuing to perform bedikot daily. She can then count her 7th clean day and immerse. (Keep in mind that if relations took place at night, that night and the following day are day 1, the day after that is day 2, etc. So if she immersed and had relations on Thursday night, the four days are Friday-Monday, Tuesday is her delayed 7th clean day, and she re-immerses in the mikveh on Tuesday night.)

If she only realizes the error after some time has passed, and more than five days have elapsed since her last bedikah, she needs to restart her clean days. She needs to wait four days from the last time she had intercourse, and then count a new seven clean days. She does not need a new hefsek taharah.

In a case where a couple had relations following an invalid mikveh immersion, acts of teshuvah (repentance), such as prayer and giving charity, would be appropriate. To avoid errors in the future, the woman should note clearly in her calendar the night of her hefsek taharah, and her expected mikveh night.

Please feel free to get back to us with any further questions.