Please note that you are posting on an English forum. Since your thread title is in German but your content is in English, I will reply to you with information in English. You may use the translation tool with your browser to translate if you need so.
Hi DIERA Rainer,
Welcome to Microsoft Q&A forum. I'm happy to help.
I strongly recommend taking a moment to double‑check all possible locations for the BitLocker recovery key following this article: Find your BitLocker recovery key.
Please note that Microsoft does not have a way to generate, retrieve, or bypass a lost BitLocker recovery key. If the key is not found anywhere, the only remaining option is to reset or reinstall Windows, which will erase everything on the encrypted drive. This is how BitLocker was designed to protect your data.
If you have your notebook remains in your computer, please check if it's in the encrypted drive or not. If it's not in the encrypted drive but in the unencrypted drive, your notebook stays safe.
If your notebook is in the encrypted drive, please check if it's synced with a cloud. If your notebook is synced, you can get it back following provider instructions.
If your notebook is in the encrypted drive and is not synced online, I regret to inform you that it's not recoverable.
I truly hope you can understand and I wish you all the best.