I'm not an expert on these things, so take this with a sizable grain of salt.
Naturally, you would need some other device that would listen for voice. Something like Alexa. You could probably do it from your phone using Google Assistant or Siri.
Your computer will also need to support Wake On LAN (WOL). Many do, though you may have to go into BIOS/UEFI settings to enable it.
Then, you just set up a routine in your assistant of choice to send a wake on lan "magic packet" to the computer, which will trigger it to turn on. I'm not certain if that's supported natively in any particular assistant, but if not you can likely find add-ons or additional software (like Home Assistant) that can make it happen.