Mt son's '97 Nissan Sentra would not start, or even turn over - no lights, etc. Took the battery in and it read 0 volts! Battery was still under warranty so brought the new one home, hooked it up, still nothing. Measured the voltage across the terminals with it hooked up as a dead short. After disconnecting the cables the battery shows 12.6 or so volts, so it isn't likely the problem. Where should I look next?

What do you mean by "dead short" ? If it had really been a dead short then you would have exploded the battery as you tried to install it. Were there any sparks from the terminals when you installed the battery?

Did you buy the proper battery? Was it a 34R?

If you installed the wrong battery and reversed the polarity of the cables, you likely blew a number of fuses, especially the real big under hood fuses. Therefore, check all under hood and under dash fuses with a test light or multimeter.

Not a dead short, just 0V across the terminals when hooked up. The hook up is consistent with the way it's been for years so I'm sure it's not installed backwards.

Thanks

Are you sure your meter is working properly? A battery won't go from fully charged to 0v that fast without some major things happening like extreme heat or explosion.

Have you checked all your fuses under the hood?

Did you thoroughly clean the battery terminals and cable connections with a battery terminal brush? I would also remove the negative battery cable and clean the connection, where it attaches to the body of the car. Examine the cables closely for corrosion and hidden breaks.

Stupid thought but I need to add something here because I have seen this happen...

Years ago I worked as a mechanic and we had someone have their car towed in. They had replaced their dead battery but the new one was no good and the car wouldn't start. I was told to go check it quickly in the parking lot and see if I could see anything outright so we could give the people an idea of what they were looking at for cost and time.

The battery cables had been installed all nice and tight but the guy overlooked one thing when he tried to save money by installing his own battery... he neglected to remove the plastic caps from the battery terminal posts.

I assume you took the caps off the posts, right?