Old Win 10 PC died, can't find Licence Key

If your old PC originally came with Windows 10 (or was upgraded from Windows 7/8 during the free upgrade period), then your license was most likely a digital entitlement (digital license) tied to the hardware, not a product key you’d see written somewhere.

Unfortunately, digital licenses are bound to the motherboard. Since yours was fried and you’re moving to an entirely new build, the license won’t automatically transfer just by signing into your Microsoft account. Logging in does sync settings and apps, but not the actual Windows activation unless the license was a retail license linked to your account.

Here are your options:

Check license type on the old drive (if still readable): If you can still boot it or mount the drive elsewhere, runto see if it says Retail or OEM. Retail licenses can usually be reactivated on new hardware, OEM ones cannot.
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If it was a Retail key and linked to your MS account: You should be able to use the Activation Troubleshooter on the new PC (Settings → Update & Security → Activation → Troubleshoot → “I changed hardware on this device”) and sign into the same Microsoft account to reactivate.
If it was OEM (came preinstalled): That license died with the old motherboard. In that case, you’ll need to purchase a new Windows 11 license.
If Windows 11 is being installed without a key: It will install and run, but some personalization options (wallpapers, themes) will be locked until it’s activated.

So, the short answer: signing in with your Microsoft account on the new desktop won’t automatically activate Windows 11 unless you had a retail digital license linked to that account.

Retail licenses (bought from stores like Best Buy or online) can be transferred to new hardware.

OEM licenses (came pre-installed on your PC) are tied to the original motherboard and cannot be legally transferred.

Is the original key OEM or retail? In addition, login the microsoft account and check if the key is there.