Hi Miriam
My name is Andre Da Costa; an Independent Consultant, Windows Insider MVP and Windows & Devices for IT MVP. I'm here to help you with your problem.
Option 1: Try disabling Secure Boot
As an initial recommendation, we suggest that you disable the driver signature enforcement by following these steps:
Click Start.
Click the Power button.
Hold the Shift key and click Restart.
Choose Troubleshoot.
Click Advanced options.
Click Startup Settings.
Click Restart again.
Once your computer restarts, press F7 on your keyboard to select Disable driver signature enforcement. Your computer will now restart automatically.
If the issue persists after disabling the driver signature verification, disable Secure Boot temporarily in your computer's UEFI/BIOS. You can contact your computer's manufacturer or check the device manual for the steps on how to do this.
Important: Modifying BIOS/ complementary metal oxide semiconductor (CMOS) settings incorrectly can cause serious problems that may prevent your computer from booting properly. Microsoft cannot guarantee that any problems resulting from the configuring of BIOS/CMOS settings can be solved. Modifications of the settings are at your own risk.
If you can't find the toggle to disable Secure Boot, a firmware update for the UEFI/BIOS may be required. This can usually be found in the manufacturer's website.
Option 2:
It might be stored in your Microsoft Account.
Sign in https://account.microsoft.com/devices
Under your device, click 'Get Bitlocker Recovery Keys'