Hello Melle rawr,
thank you for posting on the Microsoft Community Forums.
Firstly, you need to retrieve your BitLocker recovery key by following the steps in Find your BitLocker recovery key - Microsoft Support.
Since you mentioned you have lost the other Microsoft account, it is recommended recover it. For details about that, please see Help with the Microsoft account recovery form - Microsoft Support.
**Important:**Microsoft Support is unable to provide, or recreate, a lost BitLocker recovery key.
Hope it helps.
Best regards,
Lei