No response from PC

Didn't use metallic current-conducting thermal paste? This can cause the motherboard to "burn".

Look at the CPU fan connector if it is on the correct pins intended for this fan, see motherboard manual. If it is on the correct pins, disconnect it and reconnect it. When the CPU fan does not start the computer does not start, or rather it does but you should have a message at the boot post and the PC does not continue to boot.

Also if you have unplugged the power connector from the motherboard, 24 pin I believe, to do your job, unplug it and reconnect it, or press it well on each end, without too much force. It has already happened to me after unplugging and reconnecting this connector that the PC no longer starts, I had to unplug it or press on each end so that it was properly seated, not much was missing.

Do this on all the connectors that you unplugged/replugged to make the fan/heatsink change.

So I know your computer started after you finished but it doesn't take much for a poorly inserted connector to no longer work because it moved for example with expansion.

I solved it just when you replied. I honestly just did a re-do. Dismantled the heat sink, removed backboard etc, removed all the specs and built the pc again. Which for now seems to be working.

My guess is a connector wasn’t properly installed as you say. (Most likely the connector from the power button to the motherboard as i noticed it was loose during the rebuild). As you mentioned it’s likely it came loose during moving the pc to the desk.

Was worried about static electricity, but not sure How common that is? I used a wristband just in case.

Either way i’m not going to buy a Pre-Built again. Limited space, and the glue made the process much more complex.

For static electricity it's either a bracelet or touching scrap metal (not sure if the latter thing works). I always or almost touched metal before starting, never had a problem, maybe it's chance. A bracelet is better.