Yeah when fans just spin up like that, it means it isn't reaching POST. So you checked the RAM already, you need to confirm video (which is on CPU so you can't) and then CPU. You can confirm 100% again that it isn't posting one of the three required hardware components. CPU, RAM, Video and of course if the motherboard isn't let those work it goes without saying. Disconnect the HDD or SSD boot up and let it spin for 30 or so seconds like it would be getting into windows and then just push the power button. If it turns of immediately when you push it, you are posting but getting no image on screen. A system that doesn't post wont respond to a single press of a power button and needs to have it held down to hard shut down.
Since I've never seen a CPU die, if it were the CPU it could really only be from overheating from bad thermal grease to heat sink marriage. I'd check to see if that is an issue. If you removed the motherboard already, you can check the pins on the LGA to make sure none of those turned color from overheating. I had an old Pentium D AIO system in a dental office years ago where the little fan cooling the CPU wasn't nearly enough. It didn't thermally shut itself down but somehow the LGA pins blued from the heat over and made a bad connection. It's not very likely but just inspect them for any damage, ect. Chances are it is just a motherboard that flaked out due to a bad trace, a sub-par component on the motherboard or something along those lines.
A suggestion, seriously avoid AIO at all cost, because that is exactly what they will do, cost you. I've had way to many fail and they were "Medical Grade" units from these guys.
https://www.cybernetman.com/ We had to replace so many motherboards, and LCD power boards I could cry. Still have some of the little boards they use to provide their LCDs power where the cheapo capacitors died. We did our own in-house repair on those with good capacitors from Badcaps.net
EDIT: OHhh one last thing to look for, see if any of the USB ports have the plastic center broken off and possibly crossing the USB leads. I've had system that do what you are talking about due to a short from USB ports when they break. It's a lot more common than you think. Especially when a user shoves a USB drive into the new USB 4th position!