Hi,
Welcome to the forum.
Does the car attempt to crank or does all power just go off at this stage?
It could be that the alternator wasn't charging the new battery and the only way to test this is to get someone to give your car a jump start and see if the car can start with either the second battery connected or a using car battery booster.
It could also be that something like the starter system is draining all power/short circuiting and this will need looking at.
Assuming that the new battery has the necessary cranking amps (given that it is new) then a prime suspect would be the electrical systems.
Cheers..