Hello there.
If you have a Retail key, no problem at all. you'll be able to activate it without major issues.
If you have an OEM key, you'll have to activate by phone, call Microsoft and explain the fact that you change the Motherboard and you like to use your OEM key in the new one (you'll probably have to tell them that the motherboard died, because i don't know if they'll let you activate the key if you just "change the mobo because you wanted to")
Activate Windows 7 by Phone