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What happens when you turn the key?
Does the engine crank?
What do you hear?
Do you see any messages on the DIC (the small text display directly over the steering wheel)?
In general:
You've checked the connections?
The cables are tourqued to 22 ft lbs.?
Have you put a meter across the terminals with the negative lead disconnected to see the voltage the battery shows? (btw you cant leave it connected and look at the voltage reading on the DIC thats not gonna be accurate)
Have you charged the battery? (this car is pretty sensitive to battery voltage, probably a lot more than any other car you've worked on. A whole bunch of systems have to wake up and check in on the bus in the right order for it to not give you headaches) so before you do anything else start with a fully charged battery to limit the ancilarry problems.
First thing I'd check is the cables & make sure they're not shorted out somewhere. By disconnecting & reconnecting you could be temporarily fixing a short.
How does the battery tray look? Does it look really bad, corroded? How do your cables look? Start with simple stuff... clean your battery terminals... and sorry if you've already done this. Working with very little info.