Prius Won't Start. Ready Light Blinking.

12.4 is very poor for a new battery. Take it back and have them put it on a charger.
It should be 13 volts.
If after it’s been installed and it drops again, or there’s still something wrong, have the trouble codes read

 

12.4v is just fine, that’s not your problem. Do you have gas in the car?

 

Are any of the warning lights lit up on the dash?

 

If the 12v battery was removed...that resets all codes, assuming the car even powered back up considering the OP stated...... I replaced it, but nothing is happening.

Is the dash turning on or no? If not, time to verify the 12v battery connections and polarity....make sure the (-) cable is firmly fastened to the car body.
What voltage do you read at the underhood jump point in the fusebox? That's where the 12v (+) battery cable connects to the car power system.

 

Here's one scenario I've seen that could cause this. The AM2 fuse provides power to the dash and the inverter cooling water pump (among other things). The ICWP tends to fail and half the time, you'll never know it and the other half of the time, it blows the AM2. SOMETIMES, the AM2 does not completely blow, and maintains just enough continuity to allow a very low current to pass (like enough to energize a digital dash) but then, since the ICWP also tried to energize at the same time, it draws enough current to make the fuse appear open again. This is a 15Amp fuse on the driverside of the underhood fuse panel. There are spares in the fusebox. Unplug the ICWP at the single wire plug just above and toward the passenger side of the fusebox, then replace the fuse. See what happens.

 

Yeah the AM2 fuse is problematic in this car You should have a new one every few years of a good manufacturer of fuses like buss. Or something along those lines they can be taken out very quickly by the inverter water pump etc like been stated above so anytime you have electronic problems anyways hopefully everyone is checking all the fuses just to make sure we in fact have voltage and the circuits are generally good pulling a fuse out and putting it on a light table or holding it up the light if you see any cracks in that mountain shaped curvature in the few center replace it It's not worth the agony and the problem that something like this can cause You can even go as far as hooking the fuse up to your beep tester with the leads and try bending the fuse if the beep stops throw the fuse and replace it with the new one.