Were both batteries in the car? If you look where the battery goes, are there any signs of physical damage?
You could always pop into your carrier's store and ask them to try and put a battery in that they know is good, that way you can at least narrow it down being a battery issue.
If you do that, I wouldn't tell them any more than they need to know. Just say it wont turn on. Let them decide if it's covered under warranty (if the phone is even still within the warranty period). If not, time to cash in insurance, or, get a new phone somehow.