Hello kayfinch,
Welcome to Microsoft community.
When you buy a computer, the pre-installed Windows system is actually tied to the motherboard. If you replace your motherboard, the Windows license you previously had will be lost with it.
For your current situation, the best solution is to contact the person who helped you replace your motherboard at the time and ask them to help you resolve this issue.
If there's anything that's not clear, don't hesitate to let me know.
Sincerely,
Mavis - MSFT | Microsoft Community Support Specialist