Thank you for reaching out. We are sorry to hear of your frustrating situation.
During the seven clean days, it is critical to perform at least one bedikah on day 1 and one on day 7. Omitting a bedikah on any of the intermediate clean days (days 2-6) does not affect your count, but omitting a bedikah on day 1 or day 7 will usually cause a delay in mikveh.
In general, a bedikah must be performed before sunset. Eight minutes after sunset is too late to count, barring extenuating circumstances.
If it is critical to immerse as originally scheduled, please get back to us with more details to help determine if your situation warrants leniency.
Otherwise, the following day's bedikah counts as your new day 1. Your original hefsek taharah remains valid. There will be a one-day gap between your hefsek and the beginning of the seven clean days, and your mikveh immersion will be delayed by one night.
For the future, we advise either performing your daily bedikah in the morning, or setting an alarm at least a half hour before sunset so that you don't miss your bedikah.
Please feel free to get back to us with any further questions.
B'Hatzlacha!