Since this is probably an LGA 2011-3 server motherboard and you're currently using Windows 10 Pro, I'm guessing you don't want to change to Windows 11. I'm still running Windows 10 on my machines.
If you do need to download the Windows 10 ISO and start afresh as others have suggested, try this link.
https://www.microsoft.com/en-gb/software-download/windows10
You might be able to reset Windows 10 exactly as you did in the past, provided your boot drive contains a "hidden" Recovery partition.
The image below can be obtained by searching for "Disk Management" and clicking on "Create and format hard disk partitions". This should open the window below:-
Locate your boot drive and see if it's similar to the image above.
In this instance, the Recovery partition is the 9.77GB shaded area to the right of Drive (C: ). This contains a copy of all the Windows installation files.
Your C: drive will be a different size to the one above, i.e. it won't be exactly 245.72GB.
The other two hidden partitions to the left of (C: ) allow Windows to boot up normally and should
not be interfered with. It's possible your drive may contain fewer hidden partitions. This is normal.
Provided your Recovery partition contains all the drivers for your new X99 mobo, you should be able to Restore Windows back to the factory default, just as you did in the past.
A quick visit to Windows Update online will (hopefully) update any missing or out-of-date drivers. Just be aware that Microsoft may detect your new motherboard and refuse to reactivate Windows, because too many things have changed. Phone the Microsoft help line and explain your old motherboard died and you've fitted a replacement. They should the reactivate Windows.
If your boot disk does not contain a working Recovery partition, you will have to download the Windows ISO, use Microsoft's Media Creation tool to burn the ISO to a USB memory stick, change the BIOS to boot from USB, wipe all the existing partitions (and data) from the disk drive, then perform a "clean" install of Windows. Easy when you know how, difficult if you've never done it before. You're familiar with the Recovery process, so try that first.
Whatever you decide to do, back up all your personal data from the drive to another physical device (portable drive, USB stick, etc).
In addition, disconnect any other hard drives or SSDs when reinstalling Windows from USB. This stops the process from installing the hidden boot partitions to another physical drive (if any).
Good luck.