Possibility 1 (worst case scenario) - since you're not the original owner, you don't know what the previous owner did. In that case it can happen that he/she might have had experimented a bit and formatted the nand or the phone malfunctioned and somehow the nand got formatted i.e full clean no bios(download mode as in android). In this case, your phone is bricked. You can possibly bring it back if you have experience with rooting, custom rom flashing all that stuff. Plus you will need the restore bootloader file for the nexus. I had experimented this: nand formatting on my galaxy s3 showed the same behaviour. Go to the XDA-Forums and search. If you don't have any experience and don't know what I am talking about, I'd suggest taking the phone to a service center.
Possibility 2 (good scenario compared to the previous one) - battery is dead since the nexus's battery is inaccessible: the same solution here, also take it to a service centre.