This PC isn't mine, it's my co-worker's at home. He is a complete newbie, so he never attached his Microsoft account.
The motherboard was replaced with a completely identical type by the vendor, the old one kaput, as it's a HP Pavilion PC (still under warranty). Problem is the Windows is now unactivated and complains no CD Key installed.
What now?
As it is under warranty it should be the vendors responsibility to fix this. Having said that, others who have changed their motherboards have reported that calling Microsoft and explaining the circumstances has often resulted in Microsoft activating it for them. Once convinced that it's genuinely the same PC, they tend to be sympathetic.
What @Bree said! +1
You can also run whiskey plus from this forum to check for a key stored in UEFI:
ShowKeyPlus - Windows 10 Forums
Calling as in Chat or Phone Call?
Over the phone has worked before...
New Motherboard and Reactivation
I recently had to change the Motherboard of my business computer. When activation didn't work online, I simply called it in. it a mechanical response but you give them your numbers they give you back number write them down and don't lose them because they will work for you the next time you have to Activate at some later date. I actually put these numbers in a FrontdeskActivation.txt file. You shouldn't have any issues just the pain in the butt with calling in and all the numbers to get exactly right.
What happens if you upgrade the CPU and /or amount of memory on the same motherboard?
The original motherboard most likely had an OEM embedded key. Those keys are unique to that motherboard. The new motherboard either has a different OEM Embedded key, or no Embedded key. showkey will tell you what's what. Who you call likely depends on what you find with showkey. If no key is found I'd be calling HP. If it has an OEM embedded key I'd just call Microsoft.
If there is an embedded key in UEFI, I would just change the product key on the activation screen to that one.
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