Sorry to hear about that! Do you remember creating a Microsoft account on the store where you purchased it from? Even if you bought it secondhand, sometimes the shop linked it to a temporary Microsoft account they gave you so the bitlocker key might be there.
Alternatively you can also try contacting the store to see if they have the copy of the key.
Please note that this is a public forum, I will be responding you in short period of time and regret for a delay in response. As I am merely a fellow user trying to provide insight and information that may be helpful to others.