iPods don't take the vehicle voltage directly so you have to use a USB to iPod lead, and USB is 5V. Automobile adapters expect 12V and convert to 5V so you might have blown up the adapter but it should protect the iPod if it sees 24V, but this isn't guaranteed. Depending on the quality of the adapter, it might work fine at 24V continuously, or work for short periods at 24V before shutting down, or blow itself up, or allow more than 5V to the USB, as the quality of design goes down. So if you used a cheap adapter, it could have allowed more than 5V through the USB and this could damage the iPod. It might still work, but after being stressed by more than the specified maximum voltage, it might not.
I'd leave it for an hour in case any internal component got hot, then try turning it on when its not plugged into a charger. Then I'd try charging it using an approved method (USB to computer or an Apple mains charger). If neither work, you are going have to take in for repair/replacement.
