I disagree, a computer that is turned off can absolutely be compromised, thought not remotely.

An attacker who gains access to a computer could take the hard drive (generic name for the boot drive or storage drive), then infect it with malware giving the attacker remote access later, then reassemble the machine.

By definition all systems can be hacked, the more complex the system, the more ways it can be hacked.

The one time pad system, is as far as I know the only encryption system that is unbreakable, if used properly, that is because it is so simple in theory, but during actual use in critical times, the system was misused by technicians, enabling some messages to be read.