Yeah, it sounds like you killed it. Is that a clone? Usually in an overvoltage condition like what presumably occurred, the serial chip on official boards goes too. The clones use more durable serial chips (but said serial chips can't be reprogrammed like the one on the official Uno/Mega boards - not that more than a 1-2% of arduino users ever do that).
If it's got a socketed Atmega328p, you might be able to revive it by replacing that with a working '328p (you'd need a pre-bootloaded one, or another arduino or ISP programmer to bootload a blank '328p with).