If o miss a day of a prayer intended to do for 40 days , whst can I do ?
Thank you for your question.
The concept of davening for 40 days, is a known segula, and it is hard to know the “halachos” of a segula. On the surface I would say that if a day was missed, you can start from the day afterwards and count from there. In general though, it is important to remember that the main point of the segula is the emuna, that Hashem is the one who controls everything, and therefore we are beseeching Him to help us. The factor of the 40 days is only a side point. Therefore it is important to keep this in mind that the main thing is the actual davening, and the amount of days is only secondary.
May Hashem answer your teffilos quickly.