Did you run the upgrade from W8 (not from boot media)?
What activation error do you get?

Do not enter a key at any time when installing Win10 using the free upgrade offer - it will not work.
Go back to your prev. OS, activate it, and do the upgrade again. Do not clean install Win10...

Let me guess - key ends with -3V66T? That's a generic key wich does not activate Windows unless your hardware is recognized by activation server -wich it should as you were activated before clean...

Good to hear, did you contact support also?

Jamie, what error did you get before? There are two issues here, one with the server not responding, or the other where the product key is blocked.