What you actually meant to say was that the car cut off while driving,,,,not that the car will not start.
Ok, so the car cut off while you were driving, did you try to restart it, before you changed the water pump ?
Or did you try and restart it, only after you changed the water pump ?
My bet is that you tried to start it before you changed the water pump and it did not start and you noticed that the water pump needed changing and for some unknown reason to us here you decided to change it out and now that you did, the car still wont start and you want to know why ?
Suggestions ? check fuel pressure, check fuel filter, check for water in fuel, scan for codes.