Resend your licence key
Lost your key? It can be sent to you again. It is emailed to the address that bought it, and it is never shown on this page. Otherwise anyone who knew your email address could read your key.
Your key is regenerated from the payment record each time it is sent. Nothing is used up by asking.
Why it is emailed rather than shown
A page that displayed a key to whoever typed an address would be a disclosure oracle: type a colleague's address, read their key. Sending it to the buyer's own address costs you one extra minute and makes that attack pointless.
You may not need it at all
Reinstalling Windows or moving to a new machine consumes nothing. There are no activations to run out of.
If your old machine still has %APPDATA%\Heimdall3D\licence.key, copying that
one file across is the whole migration.