Assuming it is a modern computer and it has been powered off (shut down) -
the operating system on it has no bearing. You can either find some way of
utilizing some "Wake-On" function of the computer (if it has it) or you can
hook some sort of network-capable power supply to it (and set the BIOS to
come on on power restorations and cycle the power to it.) OR.. You'll have
to call someone/go turn it back on yourself. ;-)