Easiest way is to enable Restore on AC/Power Loss if your bios has that feature and use a wifi plug to do a power cycle whenever you want to start it. I believe this would also works even if the computer is powered off.
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.
+1, also as an alternative to wake on LAN, you could use some kind of hardware method.
The power button to start the PC can be removed/rerouted and hooked to an external device as well, like an Arduino or another computer with GPIO.
I had a computer with a broken power button and to start it I had to touch the two wires from the motherboard power switch connector together to simulate pressing the button. An external device could simulate pressing the power button and turn the computer on from being completely off rather than in a sleep state.
I have no experience with this myself, but I know "smart" plugs are a thing:
https://home.google.com/explore-devices/lighting-and-plugs/
for example. I'm sure you could get Alexa-integrated models as well, but I suspect Google Home devices will be more hackable.