what can I do if I missed a day of the Kabbalistic square
The square is ruined, but if there aren't good and near dates to start again from zero, and square is long, you can continue as a normal 40 days work.
The power won't be that of a square.
I never did this after I failed a square, I red that you can do the do above, but I don't have direct experience on this.
A member who had experiences may help you if you want to continue as a normal 40 days work.