Yes, if that happens you have to start again from day one the next time you can begin the square on it's respective day.
Some general tips to prevent you from messing up,
Put some kind of alarm everyday to make sure you don't forget it.
Write a note somewhere, like on your phone if you have one or in a journal or on a calander where you can keep track of which day you are on and how many vibrations you still need to do, and last but not least, make sure you have a good way of carefully counting the number of vibrations when you are doing the square.
Take it slow and don't rush the squares when you are vibrating them, as that is a sure fire way to mess up or loose count..
This might sound silly, but I keep track of the number of vibrations by using the calculator on my phone and adding +1 to the counter for each vibration. Of course a well made Satanic rosary will work just as well, but if you aren't careful you can mess up with a rosary.
Nothing is more annoying imo then messing up your moon square on day 80 or something like that.
When you make a mistake the entire work is canceled out. This is because the squares are very precise and in order to get them to work they must be performed perfectly because of the nature of the energies you are working with.
So don't take any chances and just work on them carefully.