Just a quick one which may save you a lot of time troubleshooting - you're welcome!
The 1903 update for Windows 10 is causing an issue for some of machines that are running Bitlocker. The update causes a bug in secure boot UEFI to start Bitlocker recovery on boot.
Microsoft have released an important Servicing Stack Update (SSU) for all supported Windows 10 versions and it should be rolled out to devices as part of the Windows Update process. However, in any case this hasn't happened or you have customisations like me, you can get it here:
Installing this should resolve the issue.
Applying the Bitlocker recovery key when asked, should allow the machine to boot properly. If it doesn’t, there is an option to launch "Advanced Options" and enter the recovery key again. The machine should then boot to the login screen. Yay! Once you're logged in, go get that update and install it.