Not a stupid question at all.
Pull the pc apart on a table covered with cardboard. All you'll need is 1 stick of ram, the cpu/cpu cooler, motherboard and psu and monitor. That's all. No other components or case.
Put the ram in slot #2, mount cpu/cooler, plug in the 2x power sockets to the mobo and using a small flat screwdriver (you'll need to look on mobo for directly where the pwr_on button pins are) gently touch both pwr pins. (if the power button will reach, use that).
This is called bread-boarding (just is, no idea where it got the name).
You should go through post and get either into bios or the inevitable warning that there's no OS drive.
If you don't, power down and swap the ram either with another stick, or into a different socket.
If it does boot, and remain booted, then the issue is not with the psu or ram or motherboard, but with one of the other components not yet mounted. Start with fully adding ram back. Then gpu, then storage.
If you get everything working outside the case, the issue is with the case. Check every single wire that's connected. Look for anything discolored or burnt or stripped/broken. Check the case thoroughly for any marks where wires went through or past metal. It's possible that during cleaning you pinched or shorted something, especially near any hdd cages.
Breadboarding starts with absolute minimums, building up until something fails. Takes a decent amount of time, (or can) or you might find the issue right off. If you can't post with just 1 stick of ram, there's only 3 possible components it could be. Mobo, psu, ram. You replaced the mobo, same symptoms, so it's either the ram or the psu. Borrow a stick from a friend, one you know is working. If it still doesn't post. It's the psu.
Once you eliminate the improbable, whatever left, no matter how impossible, has to be the answer.