In most situations the absence of the moch dachuk does not invalidate the hefsek taharah and you may count the next day as day 1 of your clean days.
The exception to the rule is if you performed your hefsek taharah on the same day that you began bleeding. In this case the moch dachuk is a strict requirement. Because of the 5 day minimum waiting period this case does not come up regularly, but it can occur if a woman bled briefly during her clean days. She may perform a new hefsek that same day if the bleeding has stopped, and a moch dachuk is critical in this case.
Please feel free to get back to us with any further questions.
B'Hatzlacha!