Yeah this is pretty common after a board swap. Windows basically thinks it's a different PC now, so it can't find your license.
Neil's question is the big one here, because it really comes down to where you got Pro from.
If you bought it yourself (full Pro or the Home to Pro upgrade), try the troubleshooter first. Settings > System > Activation > Troubleshoot, then "I changed hardware on this device recently". Sign in with the same Microsoft account you had on there before and pick your PC from the list. The catch is your license had to be linked to that account before the swap. If it wasn't and you've still got the key, just try entering it under Change product key.
If it came with the PC from Dell, HP etc, that's OEM and it's tied to the old board, so you're probably out of luck unless the manufacturer did the replacement under warranty. If they did, I'd call them. If you swapped it yourself you'll likely need a new license, sorry.
Don't remember which one you've got? Run slmgr /dli in an admin Command Prompt, it'll say RETAIL or OEM on the Description line.