how can i get my fiat bravo to start after replacing a flat battery with a ?

my mechanic had my bravo to do some basic work on it, and in doing so the battery went flat and could not start the car, recharged the battery reconnected to car but still would not start! realising battery would not hold charge couple of weeks later bought a new one connected to battery terminals, but car still would not start and still has,nt have you got a solution?