License key on more than one computer

I have a little problem. 
I activated my key on two Macs, mine and a friend's, 
but my friend formatted the computer, without deactivating the key in rekordbox, 
and now I can't activate it on another one, as I get the message:
 "the license key you entered , is being used for another Rekordbox account. 
You cannot use a license key for more than one rekordbox account. 
Error code: 19". 

How to proceed?